








































WAR DEPARTMENT 
OFFICE OF THE CHIEF SIGNAL OFFICER 






AIRPLANE MOTORS 


A COURSE OF PRACTICAL INSTRUCTION IN THEIR 
CARE AND OVERHAULING 

FOR THE USE OF 

MILITARY AVIATORS 

BY 

GEORGE E. A. HALLETT, A. M. E.: A. S., S. C. 


COPYRIGHTED, 1917 



WASHINGTON 

GOVERNMENT PRINTING OFFICE 
1918 







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WAR DEPARTMENT, 

Office of the Chief of Staff, 

Washington, January 10, 1918. 

The following Instructions on Airplane Motors, a Course of 
Practical Instruction in Their Care and Overhauling for the use 
of Military Aviators, by George E. A. Hallett, A. M. E., A. S., 
S. C., prepared in the office of the Chief Signal Officer, is ap¬ 
proved an<J published for the information and guidance of the 
Regular Army and the Organized Militia of the United States. 

By order of the Secretary of War: 

JOHN BIDDLE, 

Major General, Acting Chief of Staff. 

3 






A PRACTICAL COURSE OF INSTRUCTION IN THE 
PRINCIPLES, OVERHAULING, AND CARE 
OF AIRPLANE MOTORS. 


By Geo. E. A. Hat, lf. tt, A. M. E., Signal Corps, Aviation School, 
San Diego, Cal. 


INTRODUCTION. 

Object of this Course. —In the case of student flyers, it is to 
give them a practical knowledge of airplane motors sufficient to 
enable them to diagnose motor trouble when on cross-country 
flights and to make rapid and practical repairs if possible, or, 
in any event, to be able to send an intelligent message for parts, 
etc., and to explain to mechanicians nature of trouble and its 
remedy. 

In the case of the mechanician, the object will be to make an 
airplane motor man out of an automobile mechanic or machin¬ 
ist. Parts of it should be useful to give to automobile factories 
to help them train aviation mechanicians. The course consists 
of: (1) A series of lectures, beginning with principles and cov¬ 
ering a wide range of practical work as well as bench and 
block work. (2) A bench course, of overhauling unserviceable 
motors (preferably airplane motors; methods explained later). 
(3) Block course of installing motor on a test block or in a 
fuselage; placing propeller, cranking, handling switch and throt¬ 
tle to start motor, carburetor adjustment, “trouble-shooting,” 
emergency repairs, inspection or “prevention of trouble.” (4) 
If time permits, a study course in some good gas-engine book. 

5 




6 


AIRPLANE MOTORS. 


Attached will be found a list of twenty assignments in “ Dyke’s 
Automobile and Gasoline Engine Encyclopedia.” These assign¬ 
ments cover the automobile chassis in addition to motors, and 
will be found fairly satisfactory. While this book has many 
typographical errors, it is fairly practical. 

Methods of Conducting Practical Motor Course. 

The course should last from four to six weeks (18 hours per 
week), according to the amount of material or motors available. 

Three men on a motor is an ideal number. If more are put 
on one motor, the work progresses too fast and therefore does 
not “ soak in.” It will be possible to give a ten-day course, all 
day sessions, which will cover all the work. (Note. —Each 
three men are called a “ section.”) 

Always encourage the ashing of questions. 

The First Few Days. 

The first lecture should be given on the first day. Other 
lectures should be given as the work progresses. The first 
thing to do is to spread out all the tools on the benches and 
name them, so that all will know them by the same names. 
Give each section a simple motor. (Note. —This motor can be 
quite obsolete, and not necessarily complete.) 

Explain principles of disassembling motor as the work 
progresses. 

1. Find the number on the parts. 

2. Decide in which order parts should be removed. (Note.— 
Some parts are not accessible until others are removed.) 

3. Show best methods of removing cotter pins. 

4. Make marks on timing gears before disassembling. 

5. Keep shims and liners in proper place (explain impor¬ 
tance). 

6. Cleaning parts (especially oil passages). 

7. Nomenclature of parts. 

Assembling. 

Explain as follows: 

All parts must be carefully oiled, because the oil pump and 
oil pipes are now empty, and it will be best to keep the idea in 


AIRPLANE MOTORS. 


7 


mind that an engine may run as much as ten minutes before 
the pump can deliver oil to all the parts. 

Bearing nuts must be tight. Never loosen the nuts to loosen 
the bearing, but add shims or scrape it out if it is too tight. 

Explain the following: 

Making paper gaskets. 

The use of shellac and graphite with these gaskets. 

Grinding valves. (Note.— Grind by hand and lift off the seat 
between each stroke.) 

Test the valves with gasoline. 

How to distinguish the exhaust valves from inlet valves. 
(Note. —The exhaust valve stems will be darkened by heat; 
sometimes the heads are designed differently. Exhaust valve 
springs are always the strongest if there is any difference in 
the two springs.) 

Valves should never be ground more than necessary. If 
there is a fair width of seat all around, it is better to leave a 
few “ pits ” near the edges rather than to grind so much of 
the valve seating away. 

Explain valve timing in its simplest form, viz, put the gears 
together by their marks. (Note. —It is not necessary to put 
an ignition system on this engine.) 

Have the class trace the water circulation and oil circulation. 

Explain how to determine the direction of rotation of the 
motor by watching the valves, as follows: 

We know from the lecture on the first day what must occur 
inside a motor to make it run. Remember that after the exhaust 
stroke (recognized by the exhaust valve being open) we have a 
suction stroke (which we recognize by the opening of the inlet 
valve). Therefore when we turn the motor in the proper direc¬ 
tion the inlet valve will open immediately after the exhaust 
valve closes. 

The above work will consume a different length of time with 
different classes and motors, and therefore I set no definite time 
for its completion. 

When this work is done, give the class motors which are com¬ 
plete and more modern, preferably airplane motors. 

It is an immense advantage if the motor can be installed on 
a testing block or in a fuselage of an airplane and run on com- 


8 


AIRPLANE MOTORS. 


pletion of overhauling, because the class will show far more 
interest and effort under these circumstances. 

As the sections begin work on these motors, make them ob¬ 
serve all the precautions taught on the first motors, and also 
teach them how to measure and find the valve timing (closing 
position of exhaust valve) either in degrees or piston position. 
In airplane motors the valves are usually timed by piston posi¬ 
tion. 

Explain that this is a motor which is new to us, and we must 
know the valve timing in order to be able to put it together 
properly. While it is true that ordinarily we could time the 
valves by the marks on the timing gears, remember that fre¬ 
quently parts either of the gears, the crank shaft, or cam 
shaft must be replaced, and as keyways and gear teeth are not 
cut so that these parts are interchangeable our marks will be 
of no value. Therefore we must know other methods of timing 
the motor. We will time this motor by degrees, preferably by 
piston position, as we assemble it, and must know the valve 
timing. 

Explain measuring spark timing. (Find at what piston posi¬ 
tion the spark occurs.) 

Explain how to try the bearings for looseness before the motor 
is disassembled, so that we will know how many shims to re¬ 
move before assembling them. This saves time, because if it 
is not done it will be necessary to assemble the bearings before 
deciding how many shims to remove. 

Explain that they must note the gear clearance between the 
crank shaft and the cam shaft, so they will know if the bearings 
must be raised, or possibly renewed, before scraping them. 

Try the adjustment of the thrust bearings and oil-pump gears 
before disassembling for the same reasons. The thrust bear¬ 
ing of an airplane motor must hold the crank shaft so that it 
“ floats ” between the main bearings. The cheeks of the shaft 
must not be able to touch the cheeks of the bearings. 

In adjusting a thrust bearing, if there is, for example, a 3/64tli 
of an inch end play in the crank shaft, the thrust bearing should 
be so adjusted that it will hold the shaft so that 2/64 of that 
clearance will be on the side which will be reduced by wear in 
the thrust bearing. Then if the motor is a tractor, the greatest 


AIRPLANE MOTORS, 


9 


amount of clearance should be left on the side of the crank 
throw which is toward the thrust bearing, because the pull of 
the propeller will wear the thrust bearing in that direction. 

Explain the testing of the crank shaft, or, if possible, test the 
crank shaft by placing it in a lathe, between centers, and by 
using a gauge to determine to what extent the crank shaft is 
crooked. Explain that crank shafts are not used if they are 
more than 5/1000 of an inch out of line or crooked, but must be 
straightened or reground. 

Then carefully measure the journals with a micrometer. The 
journal must not be Over 5/10000 of an inch out of round. 
Crank shafts become crystallized in time, even in comparatively 
heavy engines. For instance, the big omnibus companies of 
London set a specified number of miles that a crank shaft of 
any certain make is allowed to be used, because it is found 
through long practice that if used longer than this the crank 
shafts are liable to become crystallized and wreck an entire 
motor when they break. At present, in this country, limits are 
being put on the use of certain makes of crank shafts in air¬ 
plane motors. For example, one crank shaft used in the service 
must not be used more than three hundred hours of flying time. 

Explain the alignment of the main bearings. Emphasize the 
fact that the crank shaft is limber, and if we should bear down 
on it while marking the bearings, it might be possible to mark 
a bearing which was already 2/1000 of an inch low, etc. 

The scraping of bearings should be taught, because it is nec¬ 
essary to scrape bearings in overhauling a motor, even though 
they are usually reamed when the motor is built. If possible, 
give the students some old bearing caps to practice scraping on. 
Instruct them in the use of the scraper; warn them to avoid 
starting or stopping a stroke abruptly with the scraper, because 
this leaves little notches which the scraper will jump over on 
the next cut. After cutting a series of parallel strokes to cut 
away part of a bearing, the scraper should be moved across the 
surface diagonally to smooth it up. Point out that it is neces¬ 
sary to cut comparatively lightly on the sides of the bearings. 
For example, if you are letting the shaft down into a bearing 
1/1000 of an inch, it will be necessary to cut a thousandth deep 
at the bottom of the bearing, but as the shaft is not going to 


10 


AIRPLANE MOTORS. 


be moved horizontally we will not cut any metal away at the 
top of the sides, and only one-half a thousandth half way up 
the sides. In other words, cut deepest in the bottom of the 
bearing and lighter and lighter as we work up the sides. The 
oil circulation of the motor should be systematically traced and 
all oil passages and screens carefully cleaned. 

Cutting Oil Grooves. 

If the bearings are renewed in a motor, the oil grooves in the 
new bearings should be cut in exactly the same manner that they 
were cut in the old ones. The form and location of oil grooves 
are usually experimented with in the motor factories and should 
not be altered by the mechanic who does the overhauling. 

Adjusting Bearings. 

Before deciding how tightly our bearings should be adjusted 
we must consider these things: Have we a low-pressure lubri¬ 
cating system, high-pressure lubricating system, worn bearings 
(bearings worn smooth), or newly scraped bearings? The mod¬ 
ern idea of bearings in an airplane motor is to fit them loosely, 
from two to four thousandths of an inch, and maintaining the 
oil film by forcing a large amount of oil through the bearings 
under very high pressure. If we have a high-pressure lubricat¬ 
ing system, the bearings can be and should be fitted loosely. 
For example, they should be carefully scraped and fitted to the 
shaft and then a two-thousandth shim placed in each side of 
the bearing to loosen it up. If we have a comparatively low- 
pressure lubrication system, the bearings must be adjusted 
closer, or they would hammer out. If we have newly scraped 
bearings, the shaft will touch the bearings only in compara¬ 
tively small spots. These spots wear rather fast, so the bear¬ 
ing loosens up quickly for a short time, until the shaft has worn 
the bearing to a perfect “ fit.” Knowing this, we adjust newly 
scraped bearings tighter than we would if we were simply 
adjusting bearings which we already worn smooth. 

Instruction in filing should be given at this stage as it is 
frequently necessary to adjust bearings by filing the bearing 


AIRPLANE MOTORS. 


11 


cap. Impress the fact that nuts on all bearings must be very 
tight. Point out that on a “V” type motor, and also on 
the vertical type, if the bearing cap is not pulled down very 
tightly it will shift or “work” while the engine is running, 
and it is only a matter of time until the main bearing studs 
will be crystallized and broken. 

When fitting new connecting rods or newly babbitted con¬ 
necting rods to a crank shaft, it is necessary to test them for 
alignment. One method is to put a bar through the wrist 
pin end of the rod, set the rod up on the crank shaft and 
measure from the main journal on either side up to the ends 
of this bar, to find out whether the connecting rod stands 
exactly perpendicular to the crank shaft. (Old connecting 
rods can be used for this purpose.) 

Disassembling and Reassembling Piston Pin Bearings. 

Explain methods of avoiding the straining or distorting of 
the pistons and also the necessity for these precautions. 

Most of our modern motors use the aluminum alloy piston. 
Due to the expansion of the pistons when hot, the wrist pins 
must fit tightly while the pistons are cold. And as we do not 
wish to distort the piston by forcing the wrist pin into it, 
we sometimes heat the piston in hot oil or water and then 
drop the wrist pin in, thus avoiding some chances of distorting 
the piston. 

The piston rings should be removed and replaced. Explain 
methods and precautions. Explain that the lower edge of 
the piston ring and the lower edge of the ring slot are as im¬ 
portant as the face of the ring; because the rings must be free 
in the slots and therefore the pressure and the hot gases will 
get in behind the rings, and unless the lower edge is perfect, 
will get out under the rings. In this way rings are made prac¬ 
tically useless. Rings must be returned to their own slot in 
the same position as they were before (same side up). 

Explain fitting of piston rings and particularly the adjusting 
of the gap in the ring to allow for expansion. Rings must not be 
stretched or twisted. 


12 AIRPLANE MOTORS. 

Grinding and Testing the Valves. . 

Most mechanics have a tendency to grind valves to excess. 
It is best to grind them as little as possible. In other words, 
if the valve is badly pitted do not try to remove all the pits, 
but grind until you have a good width of seat all the way 
around the valve. 

The valve springs should be tested while compressing them to 
the length which they occupy while on the cylinder with the 
valve seated. Find out what their tension is by weighing them 
on a spring scale. 

Oil up the cylinder and pistons before assembling. Be sure 
to explain that rags must never be used for this purpose. 
Use only the bare hand which can be easily freed from all 
grit, and while oiling with our hands we can detect grit u 
present. Oil the piston thoroughly, under the rings, in the 
wrist pin and all over the outside. Oil the cylinder all over 
the inside; leave no dry spots. Explain precautions in slipping 
the cylinder over the pistons. If a ring is broken, it will score 
the cylinder. See that the cylinder gasket is not doubled over 
on one corner, because there is danger of throwing the cylinder 
out of line in this way. When engines have separate cylinders, 
it- is necessary to line them up carefully with a straight-edge 
so that the inlet manifold will not have to be sprung, and be 
under strain when bolted to the cylinders. It is usual to test 
the manifolds after assembling by blowing into them by mouth 
or with air pressure; then put oil on all the joints and see if 
bubbles come through. 

The water pump stuffing boxes should be carefully packed. 
Explain the importance of polishing the shaft before packing to 
prevent excessive wear on the packing. 

Explain the necessity of putting in the largest possible amount 
of packing, because the more packing you can get into the 
stuffing boxes the looser it can be left without leaking and less 
wear and less friction will result. 

Explain that the stuffing boxes should never be allowed to 
leak, because if they do, the shaft will rust, become rough, and 
cut out the packing. 

Assemble the valve-operating gear. In high-speed motors 
it is very essential to have every part of the valve-operating 


AIRPLANE MOTORS. 


13 


mechanism working perfectly freely. Adjust the valve clear¬ 
ance. (See Lecture No. 111.) Time the cam shift. (See 
Lecture No. III.) Time the magneto and wire up. (See 
Lecture No. V.) 

Be sure to have every man in the class go through these 
processes by himself and be sure he understands it. Never 
allow them to call the process complete without checking it 
afterwards to find out exactly what results they have obtained. 
This is important, even with the best mechanics in airplane 
work. Always preach the idea of preventing trouble where 
possible. 

All the way through the process of assembling a motor bear 
in mind the necessity of having an oil supply for all moving 
parts run for the first few minutes before the oil pump can fill 
all the passages and deliver oil to all parts. 

If possible, these motors which we are just finishing over¬ 
hauling should be installed on a testing block or in a fuselage, 
and run. Then, if time and material permit, widely different 
types should be overhauled. 

Block Work. 

Actual power determinations are of comparatively small prac¬ 
tical value to an airplane mechanic, but the motor can be run 
with a propeller or a club. While installing the motor impress 
upon the class that the switch and ground wire must be con¬ 
nected before the propeller is put on. In this way the motor 
can be made safe while the propeller is on, thereby lessening 
the danger of some one being kicked. On many engines it 
is necessary to place the propeller in relation to the spark 
timing, so that the propeller will come in a position which is 
convenient for a strong pull while the magneto is in a certain 
position. In case it should be necessary to explain this, it can 
be done as follows: 

Place the propeller on the engine, but without fastening, so 
that it can be swung freely without revolving the engine. Now 
stand in the proper position to crank the propeller and hold the 
blade in the position where it will be convenient to start the 
pull. Note this position. We must have room to pick up 




14 


AIRPLANE MOTORS. 


speed before the spark occurs, to avoid back kicks. Therefore 
move the propeller down about twenty degrees and note this 
position. This is where we wish the “ break ” or spark to 
occur. Next attach the propeller lightly, so the motor can be 
turned over. Open the breaker box on the magneto, turn the 
propeller until a break occurs (it doesn’t matter in which 
cylinder). Now disengage the propeller; put it in the position 
where we said we wished the spark to occur; secure it in the 
proper manner. This simply brings the explosion in the proper 
position for cranking. 

Precaution. 

In most magnetos, when the breaker cover is removed, the 
switch no longer short circuits the magneto; therefore, while 
performing this operation of placing the propeller, it will be 
wise to remove the connection between the collector brush and 
the distributor. Or if this is not accessible, remove the wires 
from the distributor or from the spark plugs, because there may 
be enough gasoline fumes in the cylinder to cause an explosion 
and a “ kick.” 

Propeller Notes. 

Never rock the propeller as a preparation to the final pull 
for starting. Many people are hurt in this way. Place your 
propeller where you intend to start the pull, rise up on tip toes, 
and start your pull strongly. Remember you must pick up a 
great deal of speed in the first few degrees in order to “ carry 
over ” the spark. As you finish the pull or stroke, manage so 
that you will be withdrawing your hands. If the motor kicks 
back, never try to resist it; simply withdraw your hands in¬ 
stantly. Never have tools in your pockets while cranking. They 
may fly out of your pockets and be “ batted ” through your legs 
by the propeller. 

The man at the propeller and the man at the switch should 
“ sound Off ” what they are doing, so there inay be no misunder¬ 
standing. Make it a rule that the man at the switch will not 
% say “ closed ” until the switch is closed, because if he should say 
“ closed ” and then leisurely reach over to close the switch or 
make the motor safe, the man at the propeller might work faster 


AIRPLANE MOTORS. 15 

and pull the propeller, believing tlie engine to be safe, and get a 
severe kick. 

It is very important to have the throttle closed or nearly 
closed whenever the switch is open. On the testing block, crank¬ 
ing the motor with the throttle open to start it will only result 
in the motor starting violently and a possibility of injuring the 
man cranking; but in an airplane the danger is greater, because 
the machine is likely to start ahead and seriously injure the man 
cranking the propeller, as he would not be able to get out of the 
way. 

After installing the motor, make the class inspect the water, 
oil, and gas supply and also the ground wire and switch. Start 
the motor and run it slowly at first. Point out that the motor 
must be warmed up slowly and gradually to avoid unequal 
expansion and consequent cracking of parts. 

Carburetor Adjustment on the Block. 

This work is particularly valuable in training the pupil’s 
ear. The Model “ L ” Schebler carburetor is suitable for this 
work, because it is adjustable yet simple. To make the ad¬ 
justment of the carburetor as simple as possible, adjust the 
auxiliary air valve spring so that it just holds the valve against 
the seat, before starting the motor. This leaves only the gaso¬ 
line adjustments to be made. 

Then emphasize the fact that the looser the auxiliary air 
valve spring is left, the larger volume of air will pass through 
the inlet pipes to the cylinders, giving full charges of gas to 
the cylinders and therefore more speed. The limit to this is, 
that if the air valve is too loose the motor will not be able to get 
enough gasoline even with the gasoline adjustments wide open 
and also the motor will be liable to stop when throttled down, 
and “ pick up ” badly. 

Have every student go through this work himself, if gasoline 
and time are available. Teach them that they must learn to 
make the adjustments quickly to avoid overheating of the motor 
in airplane work. Then disarrange the adjustments after each 
student finishes. 

:t5840°—18-2 






16 


AIRPLANE MOTORS. 


It is well to use open exhausts (no muffler) for this work, 
so the class will become used to the noise and also the appear¬ 
ance of the exhaust under the various mixtures and conditions. 

Diagnosis of Trouble. 

This is a very practical subject and can be so taught that 
it will be very valuable not only to the mechanic but also the 
pilot. The work consists of artificially causing troubles which 
really do occur in practice on the field, then having the class 
find the troubles. It is necessary to have the motor, prefer¬ 
ably an airplane motor, mounted on a block where it can be 
run. The instructor should plan the troubles carefully, simple 
ones at fifst, and plan so there will not be two troubles giving 
the same- symptoms at the same time. 

To get the real value out of this work, first give Lecture No. 
VIII on system for diagnosis of trouble. Do not let the class 
go ahead and find troubles by inspection and haphazard 
methods, but make them reason out where the trouble is and 
why. If necessary reason out loud for them. Never make 
troubles by changing carburetor adjustments and never allow 
the class to correct troubles by changing these adjustments. 
By following this rule, you will help to combat a failing of the 
human race, viz, to try to correct all troubles by adjusting the 
carburetor. 

The following are some of the troubles we make on the 
Curtiss 8-cylinder engines. They may serve as a suggestion 
for this work. 

Ignition Troubles. 

A Bosch magneto type DBS is suitable for this work. 

Bad plug. 

Shorted spark plug. 

Loosened spark plug gasket. 

Spark plug wire off. 

Spark plug wire shorted against motor. 

Ground wire short-circuited. 

Magneto leads crossed. 

Magneto leads disconnected. 


AIRPLANE MOTORS. 


17 


Magneto brushes missing. (Collector brush, bridge brush, 
ground brush, and distributor brush.) 

Broken insulation at spark. 

Dirt in breaker. 

Various breaker troubles. (Points of breaker apart; out of 
adjustment and breaker screw loose.) 

Magneto firing on the wrong stroke. 

Safety gap shorted. 

Breaker timed wrong. (Off the key.) 

Water in magneto. 

Various shorts in the secondary. For example, drill a small 
hole in the distributor arm to the middle of the core in the 
center; insert a small wire in this hole and cover over the 
surface. 

Magneto fully retarded and key removed from driving gear 
so magneto will shift out of time. 

Magneto timed on wrong top center. 

Carburetor Troubles. 

(Note. —Schebler Model “L” is suitable for this work.) 

Remove the auxiliary air-valve spring. Block the auxiliary 
air valve open (with short piece of copper tubing). 

Remove float valve. 

Partially obstruct float valve. 

Obstruct spray nozzle, partially and completely. 

Loosen joint in the inlet manifold. 

Put bad gaskets in the inlet manifold joints. (Gaskets which 
will cause a leak.) 

Put water in the float val.ve of your carburetor. 

Cause the float to stick. 

Replace the lift-lever spring with a weaker one, so that the 
roller will not follow the cam. 

Flug the air vent in the gas tank. 

Valve Troubles. 

No clearance, or valve held open. 

Cam follower stuck (no lock washer on set screw). 



18 


AIRPLANE MOTORS. 


Defective push rod. 

Cam shaft gears meshed wrong. 

Weak exhaust valve springs. 

Cut the points off the cam-follower-guide set screw, so it will 
allow the cam follower to turn. 

Piece of wire holding valve off seat. 

Cam follower turned around. 

Water in the cylinder. Fill your cylinder full of water while 
the piston is at the bottom of the compression stroke, and if 
the water gets in several of the cylinders on the same side, have 
the class blow the water out by removing the spark plugs on 
that side of the motor and running the motor on the other 
four cylinders. This is an effective way of removing water 
from the cylinders. 

Inspection for Prevention of Trouble. 

The instructor should loosen up and disarrange numerous 
parts of the motor, propeller fastenings, gas feed, cooling and 
lubrication systems, making note of all he does. Then call the 
class out and explain that they are supposed to fly over two 
hundred miles of rocky mountains with this motor to-day and 
it will be necessary to inspect with the idea of preventing trouble , 
and make note of all they find wrong. When they are through 
compare the notes and tell the class whether or not they can 
theoretically fly safely across the mountains; then have them 
start and run the motor. 

Emergency Repairs. 

Remove suitable parts of the motor, magneto, or carburetor 
and have the class make emergency repairs with iron wire 
and tape. 

Have each section select tools and supplies which they con¬ 
sider most necessary to take along in an airplane on a long trip. 
For example, tape, assorted nuts, cotters, etc., soft wire, spark 
plugs, exhaust valve springs. Distribute the tools so there will 
be a wrench for every part of the motor, but no excess wrenches; 
try to find one wrench which will answer many purposes. Ex¬ 
plain that the most serious work necessary to prepare for would 
be the removal of one cylinder. 


TEN PRACTICAL LECTURES ON AIRPLANE MOTORS. 


(Note.—T he lecture room should be equipped with black¬ 
boards, and lectures should be illustrated by diagrams on these 
boards. Also a “ cut-away ” model of a four-cycle motor, show¬ 
ing all moving parts, magnetos, carburetors, any small parts 
which might help to illustrate the lectures.) 


LECTURE I. 


Subjects .—Principle of four-stroke cycle engine, heat loss, and 
reasons for cooling. 

(Note. —Nomenclature of parts should be taught about this 
time on the model.) 

It is of vital importance thoroughly to understand the ele¬ 
mentary principles of the motor, because once they are mastered 
we can reason out the solution of many motor difficulties. 

1. First, let us consider the burning of city gas in the open, 
unconfined. If we light the gas as it flows out of a gas jet, it 
burns with a small amount of heat and without generating any 
pressure, and with no noise. Now, if we take a pipe several 
feet long, closed at one end, and put in enough gas to fill it for 
six or eight inches, and then ignite the gas, it will burn and 
rush violently out of the open end of the pipe. It we should 
take the same amount of gas and place a plug in the pipe, the 
plug will be forced out along the pipe for a certain distance. 
Now, with the same amount of gas, if we force the plug down, 
compress the gas into one-third of its original space, and then 
ignite it, we find the plug will be driven much farther and much 
faster than before. 


19 




20 


AIRPLANE MOTORS. 


2. In the first gas engines made, gas was drawn into the 
cylinders and burned without being compressed. The result 
was it required a great deal of gas to develop a given amount 
of horsepower. In other words, the engines were very uneco¬ 
nomical. Later it was found that by compressing the gas be¬ 
fore firing they were able to obtain a great deal more power 
with the same amount of fuel. When gas is compressed it will 
burn much more rapidly. Naturally, in a motor running at the 
speed that our airplane motors do, gas must be burned com¬ 
pletely in a very short space of time in order to do any work. 
Now, we can understand that for a motor to run it will be nec¬ 
essary, first, to draw gas into the cylinder; then the gas must 
be compressed; then it must be burned and expanded and the 
burned gas cleared out of the cylinder. These four things must 
be done in any gasoline engine. 

Four-Cycle Engine. 

3. Practically all engines now used in airplanes are of four¬ 
cycle, or more correctly speaking, four-stroke, cycle type. 
This word cycle practically means a “ program.” The com¬ 
plete “ program ” of events occurring in these engines could be 
said to be composed of four “ acts ” or events. These four 
events are suction, compression, expansion, and exhaust. Each 
event requires one movement or stroke of the piston to com¬ 
plete it. The first stroke will be an outward stroke of the 
piston, drawing gas into the cylinder. This we will call the 
suction stroke. The next stroke will be an inward stroke of the 
piston, compressing the gas in the cylinder. That is the com¬ 
pression stroke. Then the gas will be ignited, and as it burns 
it will expand and drive the piston outward. This is the work¬ 
ing or expansion stroke. The fourth stroke is an inward stroke 
of the piston. A valve will be opened and the piston will force 
out all the remaining hot air and smoke left from the explosion, 
thus leaving the cylinder clean for a new charge. This is the 
exhaust stroke. 

Thus we see that we have one working stroke and then three 
idle strokes before the next working stroke. If our enginb has 
only one cylinder it will be necessary to have a large flywheel 
which will be capable of storing up energy during the one work- 


AIRPLANE MOTORS. 


21 


ing stroke sufficient to carry the piston through the three idle 
strokes. In an engine with more than one cylinder the other 
cylinders can take the place of the flywheel. To sum up: The 
four-cycle motor requires four strokes to complete the cycle. 
These strokes are first, suction; second, compression; third, 
expansion or working stroke; and fourth, scavenging or ex¬ 
haust stroke. 

4. When we compress air it becomes heated if we compress 
it rapidly enough. For instance, if we are pumping an auto¬ 
mobile tire the pump becomes warm. This is not entirely due 
to the friction of the plunger in the pump, but partly due to 
the work of compressing the air. The same is due when we 
compress gas or a mixture of gasoline and air in an engine 
cylinder. As we compress gas in the cylinder it is heated to a 
certain extent. Then we have the heat of the electric spark 
which we use for igniting the gas. The heat of the electric 
spark together with the heat of the compression is sufficient 
to “ ignite ” the gas or start it burning. Then as the gas burns 
its temperature and pressure will rise very rapidly. If the 
gas were burned in the open the temperature would go quite 
high, but when burned in a confined space the pressures are 
increased as the gas burns. The increase in pressure adds to 
the heat (as in the case of the tire pump), as does also the 
burning of the fuel. Therefore, we obtain a temperature in a 
gas-engine cylinder between 2,000 and 2,500 degrees Fahrenheit. 
You can think of this as a white-hot flame. 

Heat Loss. 

5. Let us imagine we have a cylinder and piston so arranged 
that we may compress a charge of gas and then lock the piston 
so that it can not be moved, and let us also imagine that there 
is absolutely no leakage. Now, if we ignite this compressed 
gas, our temperature and pressure will instantly run up very 
high. But should we leave it for five minutes before testing 
the pressure in the cylinder, we should probably find but few 
ounces of pressure. Now, the reason for this is that the cool 
cylinder and piston have absorbed the heat of the explosion, 
and as the heat subsided the pressure subsided also. In other 
words, the hot gases have been contracted by cooling. In other 


22 


AIRPLANE MOTORS. 


words, heat and pressure can be considered as practically the 
same thing in a gasoline engine. 

6. Now we will stretch our imagination a little further. We 
will imagine that we have a cylinder which is not only capable 
of having gas compressed in it, and then the piston locked, but 
also can be maintained at a temperature in the neighborhood 
of 2,500 degrees. This temperature, by the way, is far above 
the melting point of iron. Now, if we can compress gas and 
fire in this cylinder at this temperature, we will find that the 
pressure will remain very high in the cylinder as long as we 
maintain the temperature of the cylinder. This is because 
the cylinder and piston will not cool the hot gases after the 
explosion. The result is there is no radiation or loss of heat 
from the explosion which is commonly called “ heat loss.” 

7. With the best gasoline motors we make use of only about 
20 per cent of the heat value of the fuel, or, in other words, we 
lose i of the heat or power that is in the fuel. 

8. To show where this heat or power goes, suppose our fuel 
were 100 per cent fuel value. About 5 per cent of this is con¬ 
sumed in friction, 35 per cent is lost by radiation, mostly to the 
water jacket, and 40 per cent goes out of the exhaust valve and 
is lost. This leaves only about 20 per cent to be delivered in the 
form of power. 

9. These figures are only approximate, but should serve to 
give an idea of where all our power or heat goes. 

10. Now, after explaining heat loss, it may seem odd that we 
find it necessary to cool the motor. However, I have already 
explained that the temperature of the explosion is sufficient to 
melt an ordinary iron cylinder, and long before the temperature 
reached that point the piston would expand and stick in the 
cylinder; also, lubrication would be destroyed by the burning 
of oil and the babbitted bearings would be melted' out. Ob¬ 
viously, then, the motor must be maintained at a temperature 
sufficiently low to permit lubricating oil to work properly. How¬ 
ever, there is a limit to the temperature at which a motor can 
run, which is still lower than the limit set by the burning of the 
lubricating oil. Remember that I have explained that if we 
compress gas it becomes heated, and also we must remember 
that up to a certain point the more we compress the gas before 


AIRPLANE MOTORS. 


23 


firing the more power we obtain for a given amount of fuel. 
However, if we compress the gas to excessive pressure, the heat 
of compression will ignite the gas and will ignite it long before 
the proper time—in other words, before the piston nears the top 
of the stroke—and the result will be a tendency to drive the 
piston backwards. Suppose we have an engine designed with 
the proper amount of compression; if some portion of the cyl¬ 
inder becomes red hot or even approaches that temperature, 
the heat of this portion, together with the heat of compression^ 
will be sufficient to ignite the gas—in other words, cause what 
we call “ preignition.” 

Cooling. 

11. There are two methods used at present for cooling. One 
is by air cooling direct and the other through the medium of 
water, which is later cooled by the air. 

12. To begin with, let us remember that a square inch of 
metal will radiate or give off a certain amount of heat in a 
given time. The more square inches of surface we have on 
the outside of a motor cylinder the more heat can be radiated 
or given off in a given time. We are all familiar with the ap¬ 
pearance of a motor-cycle cylinder. It is constructed with 
numerous fins or ridges around the cylinder, the purpose of 
wdiich is to present as many square inches of surface to the air 
as is practical. 

13. Remember that the more air that passes a square inch 
of metal, in a given time, the more heat the metal will be able 
to give off or radiate. Therefore, we find large air-cooled 
motors, arranged with a blower of some description which will 
cause a large amount of air to flow by the cooled flanges on 
the cylinders, and frequently there are jackets or housings 
around the cylinder which will force the air to flow between 
these cooled flanges. This applies to motors which have sta¬ 
tionary cylinders. In the case of rotary motors, which we will 
take up later, the cylinders are passed through the air constantly 
and therefore no fan or blower is required. 

14. The other method of cooling motor cylinders is to con¬ 
struct them with a double wall and fill the space between the 
two walls with water. Then as this water is rapidly heated 


24 


AIRPLANE MOTORS. 


by the explosion inside the cylinders, we must have means of 
removing the hot water and replacing it with cool water. For 
a stationary motor, we might use a large tank of water to draw 
from, and pump the hot water from the cylinders back into 
this tank; but in airplanes and automobiles it is necessary to 
carry a small amount of water for the sake of light weight; 
therefore we must have a means of rapidly cooling this water. 

15. You all know that the way to cool a dish of hot mush 
is to spread it out thin, in this way exposing many square 
inches to the air. If we could carry a tank so arranged that 
the water can be spread out over a hundred square feet and 
not more than a sixteenth of an inch deep, this would make 
an ideal radiator as far as cooling is concerned, but would be 
far too bulky and we would lose water by evaporation. There¬ 
fore, we use radiators of various types. These radiators are 
so constructed that they spread the water in very thin layers, 
usually about the thickness of heavy paper. The water is 
enclosed in thin sheet copper, which is an excellent conductor 
of heat. Water heats the copper and the cooper is cooled by 
tbe air. In a well-designed radiator we expose a great many 
square feet of surface to the cool air. In this way compara¬ 
tively small radiators can cool a large amount of water in a 
short time. In an airplane the radiator is drvieu constantly 
through the air at a high speed, thus insuring an ample supply 
of cool air. 

16. There are two methods of taking the water from the 
radiator to the cylinder and from the cylinder back to the 
radiator, or, in other words, circulating the water. First, we 
have the system which employs a pump of some type, usually 
a centrifugal pump. This pump circulates the water through 
the water jackets at the speed which is found to be best by the 
designer of the motor. Also, we have what we call the Thermo- 
Syphon system, or, in other words, heat syphon. 

17. You all know that the hottest air in a room rises to the 
ceiling and the cooler air settles in the lower part of the room. 
This is due to the difference in weight of the hot and cold air. 
The same thing is true with water in a tank. If you fill a 
tank with warm water, very soon you will find comparatively 
cool water in the bottom while the water on the surface will 
be warm. 


AIRPLANE MOTORS. 


25 


18. In the cooling systems of motors this works out as fol¬ 
lows: The water in the water jacket surrounding the cylinder 
is being heated and is rising. It flows upward to the top of 
the radiator, and as it flows upward its place is taken by cool 
water from the bottom of the radiator; also in the radiator 
the water is being cooled, and as it cools it becomes heavier 
and settles to the bottom. In this way we have the heating of 
the water in the cylinder and the cooling of the water in the 
radiator, causing the water to circulate continually. This 
system has the advantage of simplicity and also of keeping 
the cylinder head comparatively hot even when the motor is 
run slowly. However, it requires larger water jackets and 
water passages, which mean more weight, and this is probably 
the reason it is not used in airplane work. With the pump or 
force circulation systems we find the designers taking advan¬ 
tage of the Thermo-Syphon principle in this way. Water 
is always introduced at the bottom of the water jacket and 
removed at the highest point of the water jacket, delivering 
to the top of the radiator and drawing from the bottom. In 
this way the Thermo-Syphon principle helps the pump instead 
of hindering it. 


LECTURE II. 

COMPARISON OF AIR AND WATER COOLED TYPES. 

1. The air-cooled motor can run at higher temperatures than 
the water-cooled motor, and as there is less difference in tem¬ 
perature between the cylinder walls and the heat of explosion 
there is less loss of heat by radiation or less “heat loss.” 
The air-cooled system is simpler ana sometimes lighter than 
the water cooled. It has no freezing troubles in winter, but 
as it runs hotter this means lower compression must be used, 
because on hot days, with the atmospheric temperature higher, 
the cylinder temperature may be several degrees hotter. In 
other words, the designer has no definite upper limit on his 
temperature and must make the combustion chamber of a size 
which will give a compression suited to the highest likely tem¬ 
perature. This means comparatively low compression. With 
water-cooled motors, as the water temperature reaches 212 



26 


AIRPLANE MOTORS. 


degrees (boiling point of water) the water is rapidly lost. 
Therefore there is quite a definite upper limit to the tempera¬ 
ture at which this type of motor can be run, and this means 
that a comparatively high compression can be used and better 
fuel economy obtained. 

2. When a motor has excessive compression, it will run well 
for a short time, then will lose power from pre-ignition. Air¬ 
plane motors are particularly likely to be troubled with pre¬ 
ignition, because of the fact that they are run continuously 
on full throttle, which also means full compression. 

Types of Motors. 

3. The type of motors with which we are all familiar is the 
vertical, where the cylinders are arranged in a row, standing 
vertically over the crank shaft. Motors with one, two, three,' 
four, or six cylinders nearly always are made in this type. If we 
attempted to make eight cylinder motors in this type with the 
cylinders in line, it would make a very long motor which would 
take too much room in the airplane fuselage and also the crank 
case Would have to be made quite heavy in order to be stiff 
enough. Eight and twelve cylinder engines can be built with the 
cylinders in line or vertical for marine purposes, but are not 
likely to be built in that way for airplanes. 

4. Suppose we decide to build an eight-cylinder motor. Now, 
if we can arrange the cylinders in two rows of four, side by side, 
we cut in half the length and weight of the crank shaft, cam 
shaft, and crank case, thus making the motor lighter, more com¬ 
pact, and more rigid. 

5. We have airplane motors built of still another type, where 
the cylinders are placed around a single crank pin in the same 
manner as the spokes of a wheel are placed around the hub. 
We call this the radial type. This type gives a very short and 
rigid shaft. The crank case also is a strong barrel type of case 
not divided in the center. Also it is possible with a crank shaft 
like this to use ball bearings satisfactorily. 

6. This type of motor is built both in the air and water 
cooled types. In the air-cooled form it requires no fan or 
blower, because the cylinders are so placed that the air will 
reach all of them evenly without any blower or jacket. There 


AIRPLANE MOTORS. 




is a tendency for the lower cylinders to become flooded with 
oil, but this has been satisfactorily overcome by constantly 
pumping the oil out of the crank case and not allowing it to 
accumulate there. This type of motor is hard to enclose in a 
stream-line hood and therefore has not been used on high speed 
machines. 

7. We have still another type of motor built with the cylinders 
arranged in the same manner as radial motors but operating 
differently. Up to the present, we have considered only sta¬ 
tionary cylinder types in which the cylinders remain stationary 
and kick or drive the crank shaft around. Others are known 
as rotary or, correctly speaking, rotating motors. The cylinders 
and crank case are similar in arrangement t6 the radial type. 

8. In the rotary type of engine, the crank shaft is bolted to 
the airplane and the propeller is attached to the crank case, 
just the reverse of the practice with stationary cylinder motors. 
The crank shaft then stands still and the cylinders drive them¬ 
selves around it. With this type of motor, we have to deal with 
centrifugal force. In other words, there is a strong tendency for 
the cylinders to be thrown away from the center of the engine. 
This makes a steel construction necessary, and in order to re¬ 
duce this strain as much as possible, the cylinders are built of 
steel with very thin walls of very light construction. 

9. The rotary engines combining the principles which tend to 
lighten the weight, as in radial motors, with an extremely light 
all-steel construction, are the lightest motors now in use. But, 
unfortunately, they consume so much fuel and oil that if we 
weigh them with fuel for a long flight, they are heavier than the 
stationary cylinder types; but for short flights, they remain the 
lightest. The reasons for their excessive fuel consumption are, 
first, that as they are air cooled they have comparatively low 
compression; and also that they require between ten and twelve 
per cent of the power they develop to rotate the engine, or drive 
the cylinders through the air. 

Types of Cylinders. 

10. There are two main types of cylinders used. They differ 
in location and operation of their valves, and also in the form 
of their combustion chambers. The “ L ” head type has the 


28 


AIRPLANE MOTORS. 


valves side by side in a pocket at one side of the combustion 
chamber. The valves are operated by simple cam followers or 
plugs riding upon the cam shaft which is in the crank case. 
The cams raise the followers which raise the valves, making a 
simple valve gear. This design makes room for long valve stem 
guides and springs, and there is a straight thrust on the valve 
stem and no side thrust which tends to wear the guides. 

11. The valve in the head type of cylinder has the valves 
placed either vertically or at a slight angle apart, with the 
heads of the valve seating in the head of the cylinder. This 
gives a combustion chamber that has no pockets, but the valve 
must be operated by push rods and rocker arms or else the cam 
shaft must be placed along the top of the cylinder with small 
rocker arms operating the valve direct from the cam. In this 
case the cam shaft is held in an oil-tight housing. 

12. Now, I will ask you to recall the explanation of heat loss 
in the first lecture. You will remember that a large amount 
of heat was lost by radiation, or by the interior surfaces of the 
combustion chamber absorbing heat. The more square inches 
of surface exposed to the heat in the combusion chamber, the 
more heat will be absorbed and the greater will be the heat loss. 
Evidently then, the combustion chamber which exposes the least 
surface to the heat will cause the least heat loss. This also 
works another way • the cylinder exposing the least surface to 
the heat of combustion in the combustion chamber will require 
the least cooling. This may mean reduced weight in the cool¬ 
ing system. In the case of high speed motors, it is extremely 
important to have very direct gas passages. In other words, 
the gas should be able to go straight into the cylinder and 
straight out without having to flow around any corners or 
through pockets in the side of the cylinders. The “valve-in- 
the-head ” type of cylinder gives a more direct gas passage,' 
which means such a cylinder can receive a more complete 
charge of gas at high speed, and the exhaust gases can be more 
completely expelled or scavenged. 

Spark Timing or Spark Advance. 

13. Up to the present, we have considered ignition of gas as 
occurring when the piston is at the top of its stroke. It is 


AIRPLANE MOTORS. 


29 


important to ignite the gas or start it burning at such a time 
that it will be completely burned and ready to do work by the 
time the piston starts down. If our motor is run at an ex¬ 
tremely slow speed we can ignite the gas when the piston is at 
the top of its stroke, and as the combustion or burning of the 
fuel is quite rapid, the fuel would be completely burned by the 
time the piston starts down. But, if the motor is run at high 
speed, the crank shaft will turn many degrees during the time 
that the gas is burning and if we ignite the fuel when the piston 
is already at the top of the stroke, the gas will not be completely 
burned and the maximum pressure will not be obtained until 
the piston is already very far down on the working stroke. 
Therefore we will not get all the power from this explosion. 

14. In high-speed motors the spark must occur far enough 
before the piston reaches the top of its stroke so that the fuel 
will be completely burned and the maximum pressure and heat 
obtained by the time that the piston starts down. Therefore, 
the faster our motor runs the earlier the spark must occur; 
also, it makes considerable difference to the spark timing if we 
use different shaped combustion chambers. 

15. For example, if we use the “ L ” type of cylinder with 
one spark plug at one side, the flame will have a long way to 
travel to pass through the entire charge of gas. This means 
that it requires a long time for the gas to be completely burned, 
and such a type of cylinder will require an earlier spark, or 
more spark advance, as we say. 

16. It is desirable, then, to use the most compact combustion 
chamber we can, and we frequently use the two spark plugs in 
each combustion chamber, as far apart as possible. In this 
case, lighting the gas at two points, it requires less time for 
complete combustion. 

17. The “ valve-in-the-head ” type of cylinder has the most 
compact combustion chamber of any type at present. I wish 
to state here that when we crank our motor by hand we are 
turning it over at a very low speed, and therefore we set the 
spark to occur very late, frequently after the piston has al¬ 
ready started down, because, if we did not, it would be possible 
for the explosion to drive the piston backwards because of its 
slow movement. This is known as a “ back-kick,” and is al¬ 
ways dangerous to the man cranking the motor. 


30 


AIRPLANE MOTORS, 


Multi-Cylinder Engines. 

18. You will remember that with the single cylinder engine 
we found that we had one working stroke, followed by three 
idle strokes, and a fly-wheel was necessary in order to carry 
the engine through the three idle strokes. With this engine our 
power comes in jerks. There is not a steady flow of power from 
the engine, and if there is a heavy piston and connecting rod 
moving up and down, with nothing to balance it, the motor 
causes considerable vibration. While this type of motor can 
be balanced with a counterweight on the crank shaft, it never 
works very smoothly. 

19. Now, if we build a four-cylinder engine, we will arrange 
the cylinders to fire once for every stroke of the piston. In this 
case there is a much steadier flow of power or, in other words, 
a much more even torque, and it is usually arranged so that 
there are two pistons moving upwards and two moving down¬ 
wards at the same time, thus partially balancing the engine. 

20. With the six-cylinder engine, we have still more even 
torque and better balance, and with the eight or twelve cylinder 
engine we have a very steady flow of power and comparatively 
good balance and smooth running. 

21. Now the advantages of a multi-cylinder motor over a 
motor with fewer cylinders are as follows: First, each piston 
and connecting rod is smaller and therefore lighter. These parts 
are called reciprocating parts because of their back-and-forth 
motion, and it requires a great deal of power to start and stop 
these parts in a high-speed motor. Therefore it is essential that 
they be as light as possible. When we have few cylinders of 
large diameter we have much more difficulty due "to unequal 
expansion. With smaller cylinders there is less of this diffi¬ 
culty. With a multi-cylinder engine we have little loss of speed 
and power when one cylinder fails for any reason. 

22. In all multi-cylinder engines all the cylinders fire during 
one complete cycle (2 revolutions), and nearly all of them are 
arranged so that the cylinders fire at even intervals. 

23. For example, a four-cylinder engine fires four times in 
two revolutions, or twice in one revolution, which means the 
cylinders fire one-half a revolution or 180 degrees apart. A 


AIRPLANE MOTORS. 


31 


six-cylinder engine fires every 120 degrees; an eight-cylinder, 
every 90 degrees, and a 12-cylinder, every 60 degrees. 

24. The length of the working stroke is from the top center 
to the time the exhaust valve opens. Let us say, for example, 
135 degrees. If the four-cylinder engine fires every 180 degrees, 
there is an interval of 180 degrees minus 135 degrees, or 45 de¬ 
grees, when there is no pressure to turn the crank shaft ahead, 
and the fly wheel must do it. 

25. In a six-cylinder engine the cylinders fire every 120 de¬ 
grees, which is less than the length of the working stroke, so 
the working strokes overlap 15 degrees. This means smooth 
running. In an eight or twelve cylinder engine the working 
strokes overlap still more. 

26. In all high-speed airplane motors it is quite a problem to 
cool the piston head. In fact, it is usually this part of the engine 
which limits the amount of compression we can use. The piston 
head is cooled mainly by the heat flowing through its metal to 
the comparatively cool cylinder wall. If the piston is large the 
heat has further to travel from the center to the sides, and it is 
quite difficult to keep this piston head below the temperature at 
which it would ignite a charge of gas under compression. The 
only way we can use very large-sized pistons is to reduce the 
amount of compression. By so doing the charge is less easily 
ignited by hot parts and less heat will be generated with the 
low compression, but it means less fuel economy. 

27. The advent of the aluminum alloy piston has been a help 
in the solving of this problem, because aluminum conducts heat 
about three times as fast as iron or steel, which were formerly 
used for pistons. 

28. In practice the multi-cylinder, small-bore engines are not 
necessarily more efficient or economical than the larger bore 
engines, because they expose more surface to absorb heat in pro¬ 
portion to the cubic feet of gas handled. Also, there is more 
friction in the multi-cylinder motor. 

Lubrication. 

29. Lubrication consists in introducing some substance—for 
example, oil—between two rubbing surfaces, to reduce the fric¬ 
tion and wear that otherwise would occur. No matter how 

35840°—18-3 



32 


AIRPLANE MOTORS. 


smooth a metal surface may appear to sight and to touch, it is 
in reality covered with tiny ridges and hollows, that are easily 
seen under a microscope. So when two clean, smooth metal sur¬ 
faces are placed together and caused to slide over each other, 
these little ridges engage each other, or interlock, and naturally 
some of the projections are torn loose from each piece. This 
tearing away of metal is known as wear, and the resistance to 
the tearing is known as friction. When oil or grease is put 
between the two surfaces it tills the little hollows and forms a 
thin film, or layer, that prevents the metal surfaces from actu¬ 
ally touching each other except at the highest points of the 
largest of these microscopic ridges. The result is a smaller 
number of these ridges torn Loose; therefore, less wear, less 
friction, and less heat generated. 

30. This oil film must be maintained in bearings. If it is not, 
excessive wear always results. In large, heavy duty machinery 
it was found that the oil film could be maintained by making 
the bearings large in area, so that the pressure per square inch 
would not exceed a certain point, beyond which the oil film 
would be destroyed. 

31. In airplane motors it is impossible to use bearings of 
this size on account of weight. Therefore it has been found 
possible to maintain the oil film by the use of a large volume 
of oil pumped through the bearings under very high pressure; 
but space must be left in the bearings for this oil film, and also 
it is necessary to leave space and clearance in the bearings so 
that oil can be pumped through. 

32. You can easily imagine that a heavy film -of oil could not 
be forced through a bearing which had only a thousandth of 
an inch clearance. Motors using these high-pressure systems 
frequently have the bearings from two thousandths to four 
thousandths of an inch loose, thus allowing the crank shaft to 
run on an oil film instead of on babbit metal. This means less 
friction and less wear on the bearings, but it requires a high 
oil pressure. 

33. I can now point out that in deciding how tight to adjust 
the bearings of a motor we must first consider the nature of 
the oil system. If we have a high oil pressure system, we can 
fit the bearings quite loosely, In fact, we must fit them loosely 



AIRPLANE MOTORS. 


33 


or excessive wear and excessive oil pressure will result. But 
should we have a motor using a splash system, gravity feed 
system, or very low pressure we could not leave the bearings 
loose, because if we did the bearings would hammer out quickly. 
With such lubrication systems we must adjust the bearings 
closer. 

34. Another thing to be considered before adjusting the bear¬ 
ings is whether we are simply adjusting bearings which have 
been run and are worn to a perfect bearing surface or whether 
the bearings in question have been newly scraped or fitted and 
have not worn to a perfect bearing surface. In the case of the 
worn bearings they will not loosen up rapidly, and therefore can 
not be adjusted as tightly as the bearings which have been 
scraped, have not a perfect bearing surface, and will therefore 
loosen up rapidly at first. 

35. Nearly all of our modern airplane motors use this high 
oil pressure system, generally having a gear pump in the crank 
case delivering oil to some tube running the full length of the 
crank case which acts as an oil distributor. Sometimes the cam 
shaft .is used for this purpose. In any case, we find oil ducts 
leading from this tube to the main bearings of the crank shaft, 
and here we must see to it that the oil holes in these bearings 
register with the oil holes in the crank shaft, as oil is usually 
expected to flow into the crank shaft through the arms of the 
crank, out of the crank pins and connecting rod bearings. The 
oil is thrown from the connecting rod bearings up on the piston 
and cylinder walls and also the wrist pins or piston pins. The 
older‘motors always had dip pans or splash pans, arranged 
under the crank shaft in such a way that the connecting rods 
could dip into the oil and splash it to all parts of the motor, but 
as the airplane motor operates in all positions, such systems 
proved unsatisfactory, because when in some positions the oil 
from these splash pans would flood the cylinders and combustion 
chambers, causing various troubles, such as foul spark plugs 
and carbon deposits. 

36 Our modern engines have no splash pans and have a 
pump which will keep all excess oil drained from the crank 
chamber and usually have a small oil sump under the crank 


34 


AIRPLANE MOTORS. 


case, but not connected to it in such a way as to allow the 
oil to flow up into it. In other words, the oil is pumped into the 
lubrication system and the crank case is constantly pumped 
dry so that there will be no accumulation of oil in the crank 
case, which might flood the cylinders in the case of the motor 
operating in extreme positions. 

37. Oils used in motors are usually the mineral oils. Castor 
oil has been used in some types of motors where the oil is not 
used over and over again. Castor oil, if used over and over, 
will congeal or thicken. It is useful in rotary motors because 
it is not mixed with gasoline in crank case and because of its 
high viscosity or ability to cling to the cylinder walls under 
high temperature and under the influence of centrifugal force 
encountered in that type of motor. It is finally thrown out of 
the exhaust valves and lost, which accounts for the high oil 
consumption of this type. The desirable qualities for motor oil 
are first of all that it should burn only at very high tempera¬ 
tures. In other words, it is said to have a high fire test. This 
is necessary to stand the high temperatures in the motor cylin¬ 
ders. Next, it should maintain its viscosity or body at high 
temperatures and after using it should show very small per¬ 
centage of decomposition so that it will be suitable for use 
over and over again in the motor. Also it should have a low 
carbon content so that it will not leave excessive deposits of 
carbon in the combustion chamber. As a rule, a heavier oil 
deposits more carbon than the lighter oils. Lighter oils must 
be used in very cold climates because heavier oils become too 
thick, and in tropical climates the heavier oils must be used as 
the lighter oils fail to retain sufficient body under high tem¬ 
peratures. 

38. The oil in a motor must be changed from time to time, 
that is, all the old oil from the lubricating system must be 
removed and new oil must replace it. Some of the reasons for 
this are, first, that the oil in being thrown from the crank pin 
or connecting rod bearings comes in contact with the under 
side of the piston head and becomes partly burned. In this 
way it is turned black and has a large amount of carbon in it. 
Also it becomes full of metal particles from the wear of the 
bearings; and still another reason more recently discovered is 


AIRPLANE MOTORS. 


35 


that when we are using low-grade fuels in our motors a certain 
proportion of this fuel fails to burn and mixes with the lubricat¬ 
ing oil, thus thinning the oil and impairing its lubricating 
qualities. In other words, it is poor economy to try to use oil 
too long. 


LECTURE III. 

VALVE TIMING. 

1. The valves of a motor must open and close at exactly the 
right time in order to let the new gas in and the burned gas 
out of the cylinder at the right time; also it is important to let 
a good, full, charge of gas in and to scavenge or clean out the 
burned gas as completely as possible. 

2. Beginning w T ith the working stroke: At the beginning of 
the stroke the pressure is often between three hundred and 
four hundred pounds to the square inch (approximately four 
times the pressure of compression). As the piston is forced 
down in the cylinder the gas expands and pressure becomes less 
and less. 

3. Finally a point is reached where the pressure is down to 
fifty or sixty pounds per square inch and the crank is at an 
angle where the pressure has but little effect on it, and there 
is not much to gain by keeping it in the cylinder any longer. 
There is also a large amount of cylinder wall exposed to absorb 
heat; and if the gas is kept in the cylinder, overheating will 
result. Therefore it is usual to open the exhaust valve some 
distance before the piston reaches the bottom of its stroke or the 
crank reaches “ bottom center.” 

4. There is another reason for opening the exhaust valve 
before bottom center. We wish all the pressure in the cylin¬ 
der to rush out before the piston starts up, because if it did 
not the piston would have to be forced up against it and a loss 
of power would result. 

5 The exhaust valve is kept open all the way up the ex¬ 
haust stroke and slightly after the top center so the piston can 
push out as much of the burned gas as possible. If it is held 



36 


AIRPLANE MOTORS. 


open too long, the piston will draw back some of the burned 
gas as it is started down, and if closed too soon the cylinder 
will not be completely scavenged. So we see that the closing 
of the exhaust valve must be quite accurately timed. The inlet 
valve usually opens at the same time the exhaust valve closes, 
or a few degrees later, because we wish to use the full length of 
the suction stroke. 

6. The piston goes down very rapidly in a high-speed motor 
and there will not be time for a sufficient amount of mixture to 
pass through the inlet valve to give the cylinder a full charge. 
In other words, there will still be suction, or a vacuum, in the 
cylinder at the end of this stroke. Therefore the inlet valve is 
held open for a considerable period after the piston reaches the 
end of its stroke and starts up on the compression stroke. Al¬ 
though the piston is traveling upwards, gas will continue to flow 
into the cylinder on account of the suction remaining. 

7. There is another reason why the gas will continue to flow 
into the cylinder. There is a large column of gas in the inlet 
manifold which has considerable weight, and during the down¬ 
ward stroke of the piston this column of gas attains consider¬ 
able momentum and this momentum will continue to force gas 
into the cylinder, even after there is no more suction in the 
cylinder. So we see why a motor will get a more complete 
charge of gas if the inlet is held open some distance after bottom 
center. (Note. —The faster the motor runs, the longer the valve 
is held open.) 

8. The exhaust valve opens from forty to fifty-five degrees 
before bottom center on different motors. (The crank can 
be set to this angle by using a protractor, and the piston posi- • 
tion measured while the crank is in this position, so we will 
know the correct piston position when we come to time the 
valves. It is usually more convenient to time airplane engines 
by piston position than by degrees or crank angle because they 
use no fly wheel.) 

9. The exhaust valve closes on top center or within fifteeh 
degrees after top center, varying on different motors. The 
inlet valve opens at the same time the exhaust valve closes 
or possibly five or ten degrees later. (In a few engines, the 
inlet valve opens a few degrees before the exhaust valve closes.) 



AIRPLANE MOTORS. 


37 


The inlet valve closes from thirty to fifty degrees past bottom 
center. 

10. The above timing refers to all stationary cylinder types 
of four-cycle engines now used in airplanes, but is not correct 
for the Gnome rotary engine. 

Method of Timing Valves. 

11. The process of timing the cam shaft in a motor consists 
in, first, placing the crank and piston of one cylinder in the 
correct position for the exhaust valve to close. (Note. —We 
always time the crank shaft by the closing of the exhaust valve 
because this must be more accurately located than any other 
operation that the cam shaft performs.) 

12. Second, we turn the cam shaft separately in the direc¬ 
tion in which it runs until it just allows the exhaust valve 
to close. Third, we mesh the gears or connect the two shafts 
together. If we remember this simple explanation, we are 
not likely to go wrong on the valve timing. 

13. The exact method of meshing gears or of placing the 
crank shaft and the cam shaft will vary somewhat in different 
motors, but the principle remains the same. 

14. It is very important to adjust the valve clearances before 
timing the cam shaft, because a small variation in clearance 
makes a large variation in valve timing. If we should time 
the cam shaft without previously adjusting the clearance, then 
adjust the clearance afterwards, this would throw our valve 
timing out or make it incorrect. 

15. Now I will explain in detail the process of timing the 
cam shaft on a Curtiss 0X2 motor, eight-cylinder, “ V ” type. 
First, we will adjust the valve clearance. At the present time 
these clearances are supposed to be ten-thousandths of an inch 
on each valve. Then we will remove the cam-shaft gear-re¬ 
taining screw and pull the gear partly off the cam shaft, far 
enough so that it no longer engages with the crank-shaft gear 
but still engages the key on the cam shaft. This makes it 
possible to turn either shaft independently. Now we will re¬ 
move the spark plug from No. 1 cylinder and insert a rod or 
scale and turn the crank shaft in the direction of rotation until 




38 


AIRPLANE MOTORS. 


we place the piston exactly on top center. We now have a 
point to measure from. The instruction book for this motor 
will tell us that the exhaust valve should close one thirty-second 
of an inch-pa^£ the top center. Therefore we will make a mark 
on our rod or scale just even with the top of the spark-plug hole 
and will measure one thirty-second of an inch up from this point 
and make another mark. As the book told us the valve should 
close one thirty-second' of an inch after top center, we will turn 
the crank ahead until the piston has gone down one thirty- 
second of an inch. We can now see that the crank shaft and 
piston in No. 1 cylinder are in the correct position for the ex¬ 
haust valve to close. 

16. Second, we will turn the cam shaft in the direction in 
which the cam shaft revolves until the exhaust valve is open, 
and keep on turning until the exhaust valve is just seated. By 
this I mean not until we have full clearance at the exhaust 
valve, but just until the tappet screw in the rocker arm is just 
leaving the end of the valve stem. Another method of deter¬ 
mining this is previously to insert a cigarette paper between 
the tappet screw and the valve stem and when the cam shaft 
is moved far enough to allow this paper to be slipped out with¬ 
out tearing, that will indicate that the valve is just seated. 
(Note.—A cigarette paper runs one one-thousandth to two one- 
thousandths of an inch in thickness.) 

17. Now we can see that our cam shaft is in the position 
where it is allowing the exhaust valve to close, or that the 
exhaust valve is just closed. Therefore we are ready for the 
third step, or meshing the gears. All that is necessary to do 
this is to push the gear on to the shaft, but it is possible that 
after we have placed both our shafts the gear teeth may not 
be in position to mesh. In this case we will have to decide 
whether we prefer to shift the cam shaft a fraction of a tooth 
forward or backward. Probably shifting it backward will be 
safer, because it is important that we do not allow the exhaust 
valve to close before top center. Then, having meshed the gears, 
the valve timing will be correct for the entire engine, because 
the cams are so spaced on the cam shaft that having timed one 
cam correctly the rest will follow in proper order. In airplane 
work, when we are timing the cam shaft or the magneto, we do 


AIRPLANE MOTORS. 


39 


not consider the process complete until we have “ checked ” the 
timing. The simplest way to check the exhaust valve timing 
is to crank the motor in direction of rotation until the exhaust 
valve is just closing. Find this point accurately, as I have just 
explained, then place the rod through the spark-plug hole and 
make a mark. Now, this mark represents the place where the 
valve actually does close. Then, if this is supposed to be after 
top center, let us back the engine until we reach top center. 
Then make another mark. By measuring the distance between 
these marks we know exactly where the exhaust valve is closing. 

18. The same method of checking applies to spark timing with 
the exception that we watch the breaker points instead of the 
exhaust valve clearances. 

19. Occasionally a bad cam shaft is received from the factory. 
If we had a mysterious trouble in the engine that no one could 
find the cause of, it would be well to check the valve timing in 
every cylinder of the motor, and check not only the closing of 
the exhaust valves but every operation which the cam shaft per^ 
forms. 


LECTURE IV. 

FUEL AND CARBURETION. 

1. Nearly all the fuels now in use for internal-combustion 
motors are distilled from petroleum. Alcohol is not used to any 
extent as yet. I will explain briefly the methods of distilling 
lighter fuels from petroleum. The petroleum is heated usually 
by means of steam pipes, in order to avoid danger of fire, and 
when heated to a comparatively low temperature at first the 
lighter elements come off in the form of vapor. The first vapor 
to come off when condensed will be the liquid known as ether. 
If the petroleum is heated to a higher temperature, high-test 
gasoline will come off in the form of vapor. Then, by heating 
the petroleum to a greater temperature, low-test gasoline will 
come off. Now, as to the next products, that will depend upon 
the kind of crude oil used. 



40 


AIRPLANE MOTORS. 


2. There are two general classes of crude oil or petroleum. 
Most of the eastern oil is what is known as paraffine base oil. 
Most of the western is known as asphalt base oil. In the case 
of the paraffine base, after the low-test gas, next comes kero¬ 
sene, usually in large quantities. In the case of the asphalt base, 
the next product below gasoline is distillate or naphtha. After 
kerosene or distillate we get light lubricating oils, and by heat¬ 
ing the crude oil to a still higher temperature we obtain cylinder 
oils or heavier lubricating oils, and so on through the oils, until 
we come to the tar products and vaseline. The chemists divide 
these various products into many different varieties. 

3. I will explain what is meant by high-test and low-test 
gasoline. High-test gasoline is very light and volatile, and 
evaporates very rapidly if left open, while low test is heavier, 
less volatile, and will not evaporate so easily. The gasolines 
are tested by the Baum§ gravity scale. Formerly the gasoline 
of high test would show 72 degrees on test, but at present ordi¬ 
nary automobile gasoline tests about 60 degrees. 

4. As the demands for gasoline have increased in the last 
few years, the oil refineries have been forced to mix many of 
the heavier fuels with gasoline. For instance, distillate; and 
then, in order to make the gas test properly, they have added 
some of the higher test fuels. In other words, the present-day 
gasoline may be a mixture of everything above gasoline and 
a good deal that is below and mixed in such proportions that 
the result will serve as gasoline. 

5. There is also another method of obtaining gasoline. This 
is by compressing natural gas to a very high pressure and prac¬ 
tically wringing liquid gasoline from it. For instance, after 
oil wells have been pumped dry they often yield gas which is 
useful for this purpose, and in certain districts there are gas 
wells which are useful in the same way. This gasoline is 
known as “ casing-head ” gasoline. 

6. Carburetion is the process of mixing fuel with the air. 
Gasoline will burn rapidly and cleanly only when mixed with a 
large proportion of air. Now, the problem is to mix the gaso¬ 
line and air rapidly in a comparatively small and light mixing 
device. That is the first part of the problem, we will say. 
Originally, with the first gasoline engines, the gasoline used 


AIRPLANE MOTORS. 


41 


was of high test and would combine or mix with the air very 
easily. In fact, it was only necessary to draw air over a large 
surface wetted with gasoline and the air would become satu¬ 
rated with the gasoline vapor and was readily burned in the 
motor. But the present-day gasoline will not mix in this 
manner. 

7. There are two methods for mixing gasoline with air. The 
first is to break the gas into very fine spray, and the other is to 
vaporize it by heat. Usually a combination of these two methods 
is used. 

8. I will now try to make plain the method of breaking the 
gasoline into a fine spray. Let us start with the stationary 
gasoline engine, running at a constant speed. We will have a 
straight pipe, for instance, 1£ inches in diameter, running to the 
gas engine cylinder, so that the piston will draw air through 
this pipe during the suction stroke. Then we will have a 
nozzle or jet in this pipe, placed in such a way that all air 
must pass it. We will feed gasoline to this nozzle from a float 
chamber located beside and just outside the pipe. This float 
chamber will be so arranged that when the gasoline reaches a 
certain height, for instance, even with the top of the nozzle, 
the float will close the valve and stop the gasoline from rising 
any higher. This is a simple device for the purpose of main¬ 
taining the gasoline at a constant level. With an arrangement 
like this, the gas will feed through the nozzle equally well 
with the tank full or nearly empty. Now, as the air goes 
through this pipe to the cylinder, it will pass the spray nozzle 
with considerable speed and will pick up or suck up a certain 
amount of gasoline from the nozzle. In this way the gas will 
come out of the nozzle in a fine spray and will be fairly well 
mixed with the air by the time it reaches the cylinder, and dur¬ 
ing the compression stroke the heat of the cylinder and the heat 
of compression will help unite the particles of gasoline with 

the air. , _ .„ 

9. By using a simple carburetor, such as I have described, if 
we should try to slow the motor down by closing the throttle 
valve between the spray nozzle and the cylinder, the air would 
flow past the nozzle so slowly that it would no longer pick up 
gasoline, or in any event, it would not flow rapidly enough to 


42 


AIRPLANE MOTORS. 


break up the gas, even if it should draw some out of the nozzle. 
So we see that our carburetor is not suitable for variable speed. 

10. Our next step will be to put a choke tube in this carbu¬ 
retor, or, in other words, a restricted air passage at the point 
where the spray nozzle is located. By this means we can make 
the air passage so small that even when the engine is running 
quite slowly there will be sufficient air velocity at the spray 
nozzle to pick up and break up the fuel. 

11. Having decided to restrict this air passage at this point, 
the next problem is of what form shall we make this choke 
tube. We must choose a form of choke tube which, while it 
is small enough to get the desired air velocity at low speed, 
will still permit a large amount of air to flow through. We 
find that the Venturi form of air passage is best adapted to 
this purpose. It is practically an air nozzle so formed that the 
side in which the air enters is at rather an abrupt angle, but 
the side through which the air leaves is at a gradual angle, or, 
in other words, allows the air to expand gradually on the other 
side. This has the following advantages: First, that this form 
of opening allows more air to pass through for a given size than 
other forms, and also that aS the air is expanding as it passes 
through, there is a tendency for the jet of fuel from the nozzle 
to burst apart into a fine spray. 

12. Now, having put this choke tube, or throat, as it is some¬ 
times called, in our carburetor, we will see how it works. Evi¬ 
dently our motor will run nicely at slow speed, but when we 
open the throttle to allow the motor to run very fast we will 
have trouble, because, as the suction of the engine increases, we 
find that the flow of gas from the spray nozzle will increase 
faster than the flow of air through the choke tube. This is in 
accordance with a certain law of physics. This means that our 
mixture of gasoline and air will become richer as the engine 
speeds up; richer means that there will be too large a propor¬ 
tion of gasoline for the amount of air. Therefore we must 
find some means of diluting this mixture at high speeds. We 
will make a large air opening between the spray nozzle and 
the cylinder, and we will put a valve in this opening, which will 
be held closed by a light spring. This spring will hold the 
valve closed while we are running slowly, but as the motor 


AIRPLANE MOTORS. 


43 


runs faster and the mixture tends to become richer, the suction 
will open the valve and allow enough air to come in to dilute 
the mixture and make it right, provided the spring is properly 
adjusted. This we will call an auxiliary air passage and 
auxiliary air valve. The air passage in which the nozzle is 
located should be called the main air passage. 

13. This diluting of the mixture at high speeds is one of the 
problems of carburetor work. One form of carburetor accom¬ 
plishes this by having two jets, one working on the principle 
which I have described, so that it delivers a richer and richer 
mixture as the speed is increased, and the other arranged on a 
different principle, so that it delivers a rarer and rarer mix¬ 
ture. In this way the two jets compensate each other. We will 
take up this particular carburetor later. 

14. We find many forms of carburetors, but they are nearly 
all made on these principles that I have explained. 

15. The action of the spring on the auxiliary air valve is con¬ 
sidered faulty, because when we suddenly open the throttle 
there is a tendency for the valve to.jump suddenly and move 
too far off its seat. The result is that it temporarily admits too 
much air, “ starving ” the motor. To correct this some manu¬ 
facturers use what is known as a dash pot, to steady the action 
of the auxiliary air valve. This is simply a piston working in 
gasoline, or sometimes in air, in much the same manner as the 
piston in a shock absorber. As the air or gasoline is forced to 
pass through a comparatively small hole in the piston, the piston 
can not move rapidly. Some designers of carburetors go still 
further, and in addition to using the dash pot to prevent the air 
valve from opening too far or too suddenly they use what is 
called a metering pin, which is an arrangement whereby the 
opening of the auxiliary air valve will temporarily feed an extra 
supply of gasoline, to avoid the tendency of starving the motor 
when the throttle is suddenly opened. 

16. Some carburetors have fixed jets of certain wire-gauge 
sizes, which are screwed into the carburetor and can not be ad¬ 
justed. Others have nozzles with a pin-point valve screwed in 
or out of them, to change the size of the opening and the 
amount of gasoline flowing through them. These are called 
needle valves. Some carburetors have the needle valve con- 


44 


AIRPLANE MOTORS. 


nected to the throttle by mechanical means in such way that 
the needle valve will be opened slightly as the throttle is 
opened, or, in other words, as the speed of the motor is increased. 

Carburetor Adjustment. 

17. Let it be clearly understood that when a mixture con¬ 
tains too large a proportion of gasoline it is called rich and the 
motor is said to be flooded, and when it contains too small a 
proportion of gasoline the mixture is rare and the motor is 
said to be starved. 

18. When a motor is cold it usually requires a rich mixture 
to make it start easily. This is because the gasoline does not 
combine readily with the air. In order to accomplish this 
temporary richness of mixture for starting, most carburetors 
have a device usually called a priming or flushing pin, which 
will when operated depress the float or hold the float down in 
the float chamber of the carburetor, allowing the gasoline to 
rise so high that it will overflow through the spray nozzle of 
the carburetor, thus wetting the inside of the carburetor with 
gasoline and making it easy for the air passing through to pick 
up a heavy charge of fuel. 

19. Another method is to have some form of valve between 
the spray nozzle and the place where the air enters the carbu¬ 
retor. By closing this valve we get greatly increased suction 
in the spray nozzle, thus picking up an excess of gasoline. 
Such a device is usually called a choker. It is usually operated 
from the pilot’s seat while the motor is being cranked. 

20. Most carburetors have two gasoline adjustments and an 
air adjustment; one gasoline adjustment for low speed and 
one for high speed. In addition to this we have a little set 
screw which allows the throttle valve to close more or less 
completely, according to its adjustment. This is to permit the 
motor to idle at the desired speed. 

21. Before adjusting any carburetor the motor must be thor¬ 
oughly warmed up and ignition system must be perfect. The 
valve operation and compression must be perfect and fuel must 
flow in a generous stream to the carburetor. Also the car¬ 
buretor float chamber and spray nozzle must be clear. If there 


AIRPLANE MOTORS. 


45 


are any loose joints in the pipe between the carburetor and the 
cylinders too much air will leak in and dilute the mixture. 
Therefore these joints must be tight. 

22. I will try to explain in a general way how carburetors 
are adjusted. First, all adjustments should be in a medium 
position. In other words, halfway between the most and the 
least, of either air or gas, according to their functions. The 
low-speed gasoline adjustment should be opened two or three 
turns. The engine should be started and thoroughly warmed up. 
Then we will adjust the low-speed gasoline adjustment, and after 
we have adjusted it so that we are getting the best results we 
can adjust the high-speed gasoline adjustment while the motor 
is running at high speed. Then, having our mixture right, we 
can adjust the screw which limits the closing of the throttle until 
the motor runs at the desired speed for slow speed. If we try 
to make the motor run too slow, it will stop, and if we allow it 
to run too fast it will make landing of the airplane difficult, and 
there will be danger of the machine running into and injuring 
the man who cranks the propeller to start the motor. 

Now we have made our gasoline adjustment suit the adjust¬ 
ment of the auxiliary air valve. If the auxiliary air valve is 
unnecessarily tight, our motor will throttle down nicely and 
respond nicely when opened quickly, but will fail to show the 
proper speed. Then, in that case, by loosening the auxiliary 
air-valve spring, or allowing the auxiliary air valve to open 
further, we will get a larger volume of air through the carbu¬ 
retor, giving the motor more complete charges of gas and gain 
higher speed. Accordingly, the gasoline adjustments will have 
to be reversed to suit this change in the air adjustment. In 
most of these carburetors we can make our gasoline adjustments 
suit almost any air adjustment, and in airplane work the amount 
of speed or revolutions per minute we can make is the most im¬ 
portant part of our adjustment; therefore we generally aim to 
have the air valve open as far as possible. The limit to this 
is that if the air valve opens too far our spring is too weak, and 
the motor will take air through this valve instead of through 
the main air passage when we throttle the motor down, and also 
it will fail to respond when we open the throttle quickly. So 
let us see that we have the auxiliary air valve adjusted so that 



46 


AIRPLANE MOTORS. 


it will just stay seated or closed when the motor is running very 
slowly or idle, but not tighter than is necessary. 

23. Most carburetors are provided with a means of drawing 
hot air from the outside of the exhaust pipe, for the heat helps 
to evaporate the particles of gasoline, or, in other words, helps 
the mixing of the gas with the air. 

24. If cold air only is used, or if the motor itself is cold, the 
gasoline goes into the cylinders in “ chunks,” and therefore 
it is never completely mixed with the air and is never completely 
burned. The result is that a larger amount of gasoline must 
be fed to the motor. When heat is used, or when the motor is 
hot, the heat causes the gasoline to combine more thoroughly 
with the air and less fuel can be used. The result is that if 
w T e should adjust the carburetor properly when the motor is 
cold, the mixture would become too rich when the motor was 
thoroughly warmed up, which would make poor economy and 
cause trouble. If we adjust the carburetor properly while the 
motor is hot, then the motor may be difficult to start when it is 
cold and may run badly until it is well warmed up, but this can 
be taken care of by using the choker, which I have described 
before. The main thing is to have a correct, economical, and 
clean mixture after the motor is heated up. 

25. If the mixture is too rich, it burns very slowly. The 
result is that the charge will be completely burned and the 
maximum temperature reached only after the piston has de¬ 
scended in the cylinder for a considerable distance. Thus, the 
greatest heat occurs at a time when there is a large amount of 
surface exposed to absorb it. The result is over-heating of 
the motor; also the heat will be very great at the time the 
exhaust valve opens, which would burn the exhaust valve and 
overheat the exhaust pipe. We lose power also, because the 
maximum pressure occurs late, after the piston has already 
completed part of the working stroke. 

If the mixture is too rare, first of all, we lose power and have 
excessive vibration. Next, if it is still rarer, the motor will 
“ spit ” through the carburetor. This is known as a back-fire. 
It is supposed to be caused in .this manner: This rare mixture 
also burns very slowly, so slowly in fact that there will be a 
flame remaining in the cylinder even after the exhaust stroke, 


AIRPLANE MOTORS. 


47 


and when the inlet valve opens to admit a new charge, the 
flame remaining in the cylinder will ignite the gas and the flame 
will travel down the inlet pipe to the carburetor. This is 
dangerous because it is likely to set fire to the airplane, espe¬ 
cially if the gasoline pipes are leaking slightly, near the car¬ 
buretor, which they often are. 

The Zenith Carburetor. 

26. This carburetor is used all over the world on airplane 
engines. It is of the so-called non-adjustable type. The only 
method of adjusting the carburetor is to remove the jets or 
the choke tube and replace them with other sizes. These car¬ 
buretors are fitted with the proper size jets and chokes at the 
factory where the motors are tested and never require further 
adjustment. The Zenith is of the type of carburetor having 
no auxiliary air valve but using the compensating jet prin¬ 
ciple. In other words, it is has a main jet which delivers a 
richer and richer mixture as the suction is increased and a com¬ 
pensating jet which is arranged to deliver a rarer and rarer 
mixture as the suction is increased. To make the operation 
of these two jets plain, we will suppose that I am sucking 
lemonade out of a bottle, which corresponds to the “float cham¬ 
ber,” with a straw corresponding to the main jet. The harder 
I suck on the straw, the more lemonade I can get (richer and 
richer mixture). Now, for the compensating jet. Suppose you 
pour lemonade from a bottle drop by drop into an open glass, 
this glass corresponding to the “ well ” in the carburetor. I 
would suck from the glass or “well” through a straw (cor¬ 
responding to the priming tube or cap jet). No matter how 
hard I suck I get only the amount that you drop into the glass 
and some air. As I increase the suction in the straw and cannot 
get any more gas I get more and more air, or the mixture I get 
will be thinner, rarer, and rarer. 

27. Now, I will try to explain the operation of the carburetor. 
(The names of the parts on the carburetor should be taught on 
the model at this point.) .As we come to start the motor, we find 
that the gasoline has gradually flowed through the compensating 
jet and has filled the “ well ” to a point even with the top of 

35840°—18 - 4 


48 


AIRPLANE MOTORS, 


the main jet. If the motor is cold, we must close the throttle 
to take the charge, and as the pistons can draw air only through 
the priming tube they will suck up liquid in the well through the 
priming tube until the well is empty, thus giving us a rich mix¬ 
ture for starting the cold motor. 

28. Now, if we open the switch and start the motor, it will 
run slowly with the throttle closed or nearly closed, and it 
will still be sucking only through the priming tube. But at 
the compensating jet supplies gasoline to the well and priming 
tube in very small quantities, the pistons will draw far more 
air than gasoline through the priming tubes. Therefore we 
can think of this priming tube as a miniature carburetor for 
running the motor idle. Now, if we open the throttle slightly 
the suction will be decreased in the priming tube and increased 
in the choke tube of the carburetor in the main air passage. 
Therefore the suction will naturally be transferred to the cap- 
jet. or outer jet, placed in the choke tube. We are now draw¬ 
ing this same amount of gasoline and some air from the “ well ,r 
and compensating jet, but through a new route, viz, the cap- 
jet. Then as we open the throttle a little more the air ve¬ 
locity through the choke tube will increase and the main jet' 
begin to deliver a small amount of gasoline. This main jet, you 
will remember, is the type which will deliver more and more gas 
as the suction is increased. We can call this main jet our high¬ 
speed jet. 

29. As we use the priming tube for slow running it is not 
necessary to have the choke tube very small; therefore the 
suction will not increase around the main jet to the extent that 
it does in most carburetors. Also, the main jet is not quite large 
enough to supply the gas needed at high speed and relies partly 
on the jet which is fed from the compensating jet, and as this 
latter jet delivers a rarer and rarer mixture it will offset any 
tendency on the part of the main jet to deliver a richer and H 
richer mixture; thus we will say the jets compensate each other. J 

30. This carburetor is provided with the usual screw adjust- 1 
ment to limit the closing of the throttle valve in order to make 
the motor idle at the proper speed, and also there is a small 
air adjustment which acts on the priming tube and affects the 
running of the motor only at the idling speed. 





AIRPLANE MOTORS. 


49 


31. To take a charge when the motor is cold the throttle 
should be completely closed, and when the motor is not cold it 
should be very slightly opened, because if we took a charge with 
the throttle closed while the motor was hot there would be danger 
of getting entirely too much gasoline in the cylinders or flooding 
the motor. That is, there would be such a large proportion of 
gasoline to air that it would be impossible to ignite the mixture. 

Action of Gasoline in the Inlet Pipes and Manifold. 


32. After adjusting our carburetor to deliver the proper mix¬ 

ture* of gasoline and air under all conditions, our problem is 
not yet completely solved. We have still to solve the problem 
of keeping the gas mixed. As the mixture leaves the carburetor 
it consists of air and a finely divided spray of gasoline. It 
we could see this it would resemble steam from a teakettle. 
Now, this mixture must be kept rapidly moving or it will con¬ 
dense ; that is, the liquid will gather together in puddles in all 
the low places on the way to the cylinders. . . .. . 

33. One method of avoiding this condensation is by allowing 
hot air to flow to the carburetor or by heating the intake main- 
fold from the outside so that the walls will be sufficiently hot 
to vaporize the liquid fuel which comes in contact with these 
walls It is important to have the manifold of the same inside 
diameter as the outlet of the carburetor, because if it is larger 
the mixture will flow more slowly than it would in the car¬ 
buretor and condensation will be the result. Allowing hot air 
to flow through the carburetor helps the gasoline spray to evap¬ 
orate or combine with the air, but we must remember that it 
the mixture is heated too much it goes into the cylinder in a hot 
and expanded condition and, therefore, we don’t get as much 
of it in the cylinder to compress. In other words, we do not get 
a good, full charge, and there is less oxygen to the cubic foot 

of the heated mixture. , „ , „ .. 

34 Another problem connected with the handling of the 
nixture of gasoline and air is that with certain motors there is 
a tendency for the gas to reciprocate in the manifold, or in 
other words, to jump back and forth. This is especially true 
in a six-cylinder vertical motor with the usual firing order. 


50 


AIRPLANE MOTORS. 


The gas will be drawn toward one end of the engine by one 
cylinder and next it will be drawn in the opposite direction 
by a cylinder in the other end of the motor; thus a very hard 
condition has to be overcome and causes uneven gas distribu¬ 
tion and condensation. The same condition exists in eight- 
cylinder “ V ” type motors if a single carburetor is used, be¬ 
cause there is a variation in the suction from the two sides of 
the motor and a tendency for the gas to reciprocate between the 
two sides. 

35. Most of the modern airplane engines of six-cylinder 
vertical and eight and twelve cylinder “ V ” type motors use 
a divided inlet manifold and a duplex or double-barrel car¬ 
buretor. This effectually prevents any possibility of the gas 
reciprocating from one half of the motor to the other, because 
they get their gas from separate sources. The “ duplex ” 
carburetors have a single float chamber to supply the two sets 
of jets. 


Things to be Remembered About Carburetors. 

36. It is impossible to get a correct adjustment on a carbure¬ 
tor unless everything else about the motor is right. If a car¬ 
buretor seems to need very exact adjustment, or, in other words, 
the motor seems to be sensitive, it is a pretty sure sign that 
something is not quite right. For instance, the ignition may be 
poor, or there may be air leaks in the inlet manifold or dirt in 
the carburetor, and under these circumstances no one can get a 
good adjustment. If a carburetor drips gas—that is, seems to 
leak—it may be that the float is set too high and shuts off the 
gasoline only ‘after it is high enough to overflow through the 
spray nozzle. Or it may be that there is dirt under the float 
valve, which keeps it from seating properly. It may be a de¬ 
fective or leaky float valve. Frequently when the carburetor 
drips gasoline there is nothing the matter, and the gasoline 
dripping out is simply due to the fact that after we stop the 
motor the gas which was in the manifold or inlet pipe will con¬ 
dense and drip out through the carburetor. Therefore, when 
our carburetor leaks we must notice whether or not it occurs as 
soon as we turn on the gasoline before starting the motor, in 


AIRPLANE MOTORS. 


51 


which cage it would be trouble with the float, or whether it only 
leaks after running the engine, in which case it would be due to 
condensation and unavoidable. 

37. Back-firing in a carburetor may be caused by too rare 
mixture or by an extremely rich one. Usually, however, it is a 
rare mixture. Either mixture can burn so slowly that there 
will be enough flame left in the cylinder, even after the scaveng¬ 
ing stroke to ignite the new incoming charge of gas when the 
inlet valve opens. Sometimes the flame will reach the carbure¬ 
tor or make a noise. A leaky inlet valve can also cause a back 
fire if it leaks badly enough or sticks open. An engine can 
“ pop ” from two causes, either from back fire in the carburetor, 
which would indicate lack of gas or too much air, or possibly 
inlet valve trouble. The other cause for “ popping ” can be a 
very rich mixture or a very late spark. But there is-a great 
difference in these two, because in the case of the former the 
pop will occur in the carburetor, and in the case of the latter it 
will be in the exhaust ports or exhaust pipes. So it is neces¬ 
sary to determine where the pop is occurring before attempting 
to find the cause. 


Cold-Weather Starting. 

38. In cold weather many people prime a motor with gasoline 
direct in the cylinder to help start it. There are objections to 
this method. For instance, if the motor fails to start on the 
first priming we are likely to find that we are losing compres¬ 
sion, because the gasoline has washed the oil off the piston 
ring’s, and when the motor does finally start there will be danger 
of scoring or scratching the cylinder walls on account of the 
oil not being there to lubricate. 

39 A better method of priming the motor is to turn it over 
as fast as you can with the switch safe or closed and the throttle 
half open, and while this is being done, completely stop the 
auxiliary air opening with one hand and nearly stop the main 
air passage with the other. This will cause high air velocity 
and suction at the spray nozzle. This method gets the gas into 
the cylinder in the form of a vapor or spray. The result is the 
motor is more apt to start and there is less danger of scoring 
the cylinders. Frequently it is possible to know when you have 



52 


AIRPLANE MOTORS. 


a charge in the motor, because you can see the vapor coming 
out of the exhaust pipes by looking very closely. This applies 
to the case where the motor is cold. If the motor is hot there 
would be a certain amount of vapor present from other causes. 
Another good method of priming is to squirt gas through a 
cock in the top of the intake manifold or at the highest point 
of the manifold. In this way the gasoline will spread over the 
interior walls of the pipe and expose a large wetted surface for 
the air to pass over. In extreme cases it is a help to saturate a 
piece of rag with “ ether ” and hold over the air intakes of the 
carburetor while taking a charge. The motor can be started in 
this way when it refuses to start in any other way. 

40. Nine times out of ten when the motor seems to have 
carburetor trouble the trouble is somewhere else. It seems as 
though the human race is prone to blame the carburetor for 
all troubles, possibly because it is the easiest thing to “ monkey ” 
with. Many times I have seen people adjusting a carburetor 
to correct a trouble caused by the magneto being wired up 
wrong. If the engine is missing or acting badly in any way 
and we believe the carburetor is to blame, we can find out or 
prove it in the following manner: If we have a carburetor of 
the auxiliary air-valve type, simply run the engine at the speed 
where it is giving trouble and raise the air valve slightly off 
its seat with one finger, thus temporarily making the mixture 
rarer; and if this does not correct the trouble try holding the 
hand partially over the air intake to make the mixture tem¬ 
porarily richer. If neither of these operations stops the miss¬ 
ing or trouble, the fault absolutely is not in the carburetor, and 
if raising the air valve or thinning the mixture would seem to 
help the engine, that might only mean that there was dirt under 
the float valve, allowing gasoline to rise too high in the float 
chamber, and would not necessarily mean that the adjustments 
of the carburetor should be changed. It might also mean that 
we should have had more cold air and less heated air. If con¬ 
ditions seem to be improved by putting our hand over the air 
intake or making the mixture richer, that might mean that we 
had air leaks in the inlet manifold, no air vent in the gas tank, 
poor flow of gasoline to the carburetor through feed pipes, water 
in the gasoline, dirt in the spray nozzle, or weak exhaust-valve 


AIRPLANE MOTORS. 


53 


springs. The chances are that there would be no real necessity 
for altering the carburetor adjustments to feed more gas. 

41. Remember that we must not leave gasoline in the float 
chamber of a carburetor if the engine is not going to be run 
for a week or so, because some grades of gasoline when evapo¬ 
rating in a carburetor under these circumstances will leave a 
deposit which resembles wax or soft soap, and naturally when 
gasoline is turned on and the engine started, this deposit will 
clog some of the fine gasoline passages. In any case it is wise to 
inspect and clean the carburetor after the motor has been 
standing for some time without running. 

42^ Make it a rule never to connect the feed pipe to the carbu¬ 
retor without first allowing gasoline to run through the pipe to 
clean the pipe or flush it out and to prove the volume of the 
flow. If there is any doubt as to the amount of the flow through 
the pipes being sufficient you can prove it in this way: Sup¬ 
pose your motor burns ten gallons of gasoline per hour. You 
should not be satisfied with the flow of gas unless it will flow 
twice that fast, or two gallons in six minutes. When running 
gas through the pipe for the purpose of cleaning hold the end 
of the pipe as low as possible to insure all heavy dirt and water 
coming out of the end of the pipe, and when testing the amount 
of flow try to have the end of the pipe at the same height as 
where it connects with the carburetor in order to make it a 
fair test. To clean out the carburetor, we should begin at the 
gasoline tank and see that there is a good, air vent in the tank, 
because gasoline can not flow out of the tank unless air is able 
to come in and take its place. (Note. —This refers to gravity 
feed.) 

43. Then we should disassemble and clean out the float 
chamber, inspecting the float mechanism to see that nothing 
is working loose. We should clean out the strainers, wherever 
they may be located; clean the jets. A special socket wrench 
should be used to remove them for cleaning. Always remove 
one at a time, clean it and replace it firmly before removing 
another. See that a gasket is on it, but don’t change the thick¬ 
ness of the gasket as this would affect the flow from the jet. 
Sand, water, or rust flakes are likely to be found in the jets. In 
the Zenith carburetor there are plugs under the jets which 


54 


AIRPLANE MOTORS. 


make them accessible. These plugs also have a cup in them 
which catches dirt, and they should be cleaned out before being 
put back in place. 


LECTURE V. 

MAGNETOS. 

1. In speaking of electric currents, we speak of the flow and 
the pressure in much the same way in which we would speak 
of the amount of water flowing through a pipe or of the pres¬ 
sure of water in .the pipe. In electricity the amount of flow 
is measured in amperage, or number of amperes; and the pres¬ 
sure is called voltage, or we speak’ of the number of volts in a 
circuit. Then a high-pressure current would be called a high- 
voltage current. Low pressure would be called low voltage. 
We also use the expressions high tension and low tension. For 
ignition in a motor we require high-pressure or high-tension 
current, which will be able to force its way through the space 
between the spark-plug points. A high-tension magneto does 
three things: it generates a current, “steps it up,” and dis¬ 
tributes it, or “ hands it out ” to the proper cylinder at the 
proper time. The so-called low-tension magneto generates a 
low-tension current only and has a simple armature composed 
of a soft iron core with a single winding of wire. 

2. The high-tension magneto has a double-wound armature. 
It is constructed with a soft iron core, then a primary winding 
composed of a few turns of coarse wire next to the core. Out¬ 
side this comes a secondary or outer winding composed of very, 
fine wire and having perhaps a thousand times as many turns as 
the primary winding. (Note. —The names of the parts on a 
model should be taught at this point.) As the armature re¬ 
volves between the ends of the magnets or in the magnetic 
field a low-tension current is generated in the primary winding. 
We call this the primary circuit (primary circuit not useful 
for ignition), and by suddenly stopping this current a strong 
wave of high-tension current will be “ induced ” in the outer 
or secondary winding. This high-tension current is collected by 



AIRPLANE MOTORS. 


55 


the collector ring and carried by the collector brush to the 
distributor where these waves of current, which we call 
“ sparks,” are handed out to the right cylinder at the right time. 

3. Now, in order suddenly to stop or break this primary cur¬ 
rent it must flow through the interruptor or breaker. This 
device makes contact for a short time and allows the current 
to flow through, then quickly separates the contact points, thus 
breaking the circuit. Now, when a current is flowing through 
two points, it possesses a certain amount of momentum or speed 
and when the points are separated it tries to jump the gap and 
keep flowing a fraction of a second longer in this. way. But if 
it does continue to flow we will not have made a quick and com¬ 
plete break in the primary current and the spark will be weak, 
and this flowing after the points are separated (sparking) will 
burn away the platinum breaker points, which means that fre¬ 
quent adjusting and filing will be necessary to keep them true. 
This tendency of the current to keep flowing after the break 
occurs is taken care of or corrected in the high tension magneto 
by a device called the condenser, which is capable of temporarily 
absorbing the current, which would otherwise try to jump be¬ 
tween the points when they are separated. When the condenser 
of a magneto fails, we find the breaker points burning or pitting, 
and we find the spark becoming weak. An electric current must 
always make a complete circuit or round trip, returning to the 
place it starts from. I will now explain the routes of the cir¬ 
cuits in the magneto. You will see that we have two complete 
and separate circuits in a high-tension magneto. 

The Primary Circuit. 

4. The primary current is generated in the inner or primary 
winding armature and flows from one end of this winding to 
the breaker and condenser, from there to ground or frame of 
the engine and through the frame of the magneto to the core 
of the armature. The other end of the primary winding is con¬ 
nected or grounded on this core, so in that way the current re¬ 
turns to the place where it started. When the breaker points 
separate, the current no longer flows through them but is ab¬ 
sorbed by the condenser. 


56 


AIRPLANE MOTORS. 
The Secondary Circuit. 


5. One end of the secondary winding is grounded to the core 
of the armature. The current is induced in this winding at the 
time of the break in the primary circuit and flows to the col¬ 
lector ring and through the collector brush, through the con¬ 
nection between the collector brush and the distributor, and 
from the distributor to the central or insulated electrodes of 
the spark plug; from there it jumps to the outer or grounded 
portion of the spark plug through the frame of the motor and 
magneto back to the core of the armature, completing the cir¬ 
cuit. This secondary circuit is of very high pressure, some¬ 
thing like 30,000 volts, and must therefore be very heavily 
insulated. If one of the wires running to the spark plugs 
should be broken and the ends separated, there would be no 
outlet for this particular wave of secondary current, and it 
would jump to the ground through the easiest channel. Usually 
it would jump through the insulation of the armature, punc¬ 
turing or ruining the insulation. To avoid this, a safety valve 
is provided, which is called the “ safety spark gap.” This is a 
gap provided on the magneto which has the points separated 
a great deal farther than the points of the spark plug, but 
close enough so that the resistance will be less than the resist¬ 
ance of the insulation in the armature. In other words, it 
will be easier for the spark to jump the safety gap than to 
jump through the insulation in the armature, but it will be 
much harder to jump the safety gap than the gap in the spark 
plugs. Then if we find a spark occurring in this safety gap, 
that should tell us that the secondary current finds no way to 
get to ground. These safety gaps are enclosed in fine brass 
gauze or screen to prevent them from igniting any gasoline 
vapor which might be present under the hood of the engine. 

6. When we wish to stop the magneto we can not use a clutch 
to disconnect it so it will no longer turn, because when we let 
the clutch in of connect the magneto up again it would be out 
of step, or out of time with the motor. The method for stopping 
the magneto is to “cut out” the breaker. In other words, we 
take the primary circuit and connect it to the ground before 
it reaches the breaker. In this way the primary current is 


AIRPLANE MOTORS. 


57 


not broken or ruptured by the breaker, and therefore we get 
pactically no secondary current. 

7. In the Bosch high-tension magneto the armature gener¬ 
ates two waves of primary current per revolution. It is neces¬ 
sary to have the break in the primary occur at the time when 
the current is the strongest, and this will mean at the highest 
point of the two waves I have referred to. In other words, 
the correct time to have the break occur is when the armature 
is just leaving the pole pieces on the ends of the magnets. 
The exact measurement between the armature and pole pieces 
varies with different magnetos for engines of different number 
of cylinders, but ordinarily this position of the armature occurs 
when there is an air space of one-eighth of an inch between the 
armature and the pole pieces. As airplane motors are run most 
of the time at full speed there is seldom any necessity for chang¬ 
ing the time of^the spark, but on some machines the spark is 
retarded to make the cranking of the motor safer. In all ignition 
systems, the spark is advanced or retarded by changing the time 
of the break in the primary circuit. In the high-tension mag¬ 
neto this is accomplished by shifting the cams that operate the 
breaker. In the Bosch magneto, if the breaker housing that car¬ 
ries these cams is in the fully advanced position, the break will 
occur with the armature in the ideal position that I have spoken 
of, but if we retard the spark the break occurs at a time when the 
armataure has moved away from the pole pieces a considerable 
distance, perhaps half or three-quarters of an inch. The result 
is that this type of magneto gives the strongest spark in the 
full advance position and a weaker spark in the full retarded 
position. Sometimes the spark in this position is so weak that 
the motor can not be started on it. It is, therefore, well to start 
the engine with the spark as far advanced as we can have it 
without making the motor kick back. Some magnetos overcome 
this difficulty of having a weak spark in the retarded position by 
rocking the magnets with the breaker housing, or, in other 
words, moving the pole pieces at the same time we move the 
breaker cams, so that the break will occur with the armature 
in the ideal position anywhere from full advance to full retard. 
The Eismann, Mea, and Dixie magnetos are of this type. Some 
magnetos are arranged so that there are four positions of the 


58 


AIRPLANE MOTORS. 


armature or rotor, per revolution, in which conditions are right 
to have the break occur. These magnetos then deliver four 
sparks per revolution (of the magneto), and such magnetos run 
at one-half the speed of the other types. 

Care of the Magneto. 

8. First of all, do not overoil the magneto. Clean oil would 
not do very much harm in a magneto, but oil as we know it 
around a motor is never clean. It always contains some carbon 
or metal particles which make it a fair conductor of electricity, 
and for this reason it can short-circuit a magneto. Also, if we 
had oil between the breaker points it would string out when 
the points separate and supply a path for the current to flow 
through, thus preventing a quick and complete break in the pri¬ 
mary circuit. The breaker mechanism of all, magnetos is ar¬ 
ranged to run without oil and should not be oiled. The ball 
bearings on the armature and distributor bearings must be oiled, 
but only every thousand miles or once a week, and then with a 
few drops of high-grade, light oil. In cleaning the distributor 
we sometimes find the contacts or the segments blackened, and 
it becomes necessary to brighten them up. This should be done 
with brass polish, pumice-stone powder, whiting, or crocus cloth, 
but never with sandpaper or emery cloth, no matter how fine 
they are, because the grit from these cloths would become em¬ 
bedded in the soft insulation material of the distributor and 
cause scratching or tearing. In many magnetos there are nu¬ 
merous carbon brushes which are apt to become glazed, and 
when found in such condition they should have the glaze scraped 
off, leaving them dull. All these carbon brushes have light 
springs behind them. Never stretch these springs to cause a 
firmer contact because it is unnecessary, and if the contact is 
made firmer the carbon brush will wear faster, spreading a streak 
of carbon powder around the part it touches, and will be likely 
to short-circuit the magneto in this way. After cleaning the 
distributor, oil can be rubbed around it with the finger; then 
the oil should be wiped lightly with a clean rag, leaving only an 
oily appearance, which is about the right condition. If too much 
oil is left in the distributor, it will form a paste with the powder 


AIRPLANE MOTORS. 


59 


from the carbon brushes and conduct the current from one 
contact to the next. If the distributor is left too dry, as it 
would be after washing with gasoline, cutting and tearing 
would result. If the magneto is driven by a gear direct on the 
armature shaft, great care must be taken to be sure that the 
gears mesh properly. If the gears mesh too loosely, they can 
jerk back and forth and strain the magneto, but if the gears 
mesh too tightly the bearings on the armature shaft of the 
magneto will be ruined in a very short time, putting the mag¬ 
neto out of commission. Gear teeth should never “ bottom ”— 
that is, the points of the teeth should never touch the bottoms 
of the spaces in the other gear, but a clearance of from 1/04 to 
1/32 of an inch must be left between the gears. 

9. When inspecting or cleaning a magneto never remove the 
end plates which carry the armature bearings, because they are 
accurately fitted to hold the armature just .002" from the pole 
pieces on all sides. Never remove the magnets from the mag¬ 
neto or the armature, because if this is done some of the magnet¬ 
ism or “ pull ” will be lost from the magnets. These two opera¬ 
tions should only be performed by a well-equipped magneto 
expert. It is not necessary to do these things in the field. 

Testing the Magneto. 

10. First, suppose we have the magneto on the bench. By 
taking hold of the magneto shaft where the coupling or gear 
would go, turn the magneto in its proper direction (usually 
indicated by an arrow on the magneto). Notice how much 
pull or resistance it offers as you turn it over. This should 
feel a good deal like the compression in a motor; and by feeling 
a magneto in this way which is known to be up to full strength, 
and then feeling your magneto, you can form an idea as to the 
condition or strength of the magnets. If the magneto is on the 
engine and we wish to test it to see if it throws a spark, the 
best place to test it will be from the collector brush (which 
collects the secondary current from the armature) and make a 
spark jump from this brush holder to the frame of the engine 
by holding a screw driver against the engine and leaning it so 
it passes close to the brush holder. The magneto should throw 


60 


AIRPLANE MOTORS. 


a spark at least an eighth of an inch while the engine is being 
cranked by hand, but we must remember one important thing. 
Whenever conditions are right for the magneto to deliver a spark 
at the collector brush it will also deliver sparks to the spark 
plugs in the cylinders. This would make the cranking of the 
motor dangerous; therefore we must either remove all the wires 
from the spark plugs, which would be quite a job, or the more 
convenient method is to remove the connection between the col¬ 
lector brush and the distributor, in order to safeguard the man - 
who is cranking. Also remember that the switch must be open 
when we are testing the magneto. 

11. If we should fail to find the spark, the first thing to do is 
to remove the ground wire or short-circuit wire from the mag¬ 
neto, because if there is any defect or short-circuit in the ground 
wire or in the switch it would have the effect of continually 
shortening the magneto even when the switch is open. So if we 
get a spark only after disconnecting the ground wire, we can , 
be sure that the ground wire in some way made contact with the 
frame of the engine when the switch is open. 

12. Let us clearly understand that when the switch is 
“ closed ” the primary current from the magneto flows through 
the switch into the ground or frame of the engine instead of 
flowing through the breaker of the magneto, and therefore we 
get no spark in the cylinders, and we say that the motor is 
safe. When the switch is “ open,” that means that there is no 
path for the primary current to flow directly to the ground and 
it must flow through the breaker. In this way a spark will be 
delivered. But should our ground wire chafe against some 
metal part in the airplane so that the wire itself makes actual j 
contact with this metal, the condition would be the same as 
having the switch closed all the time. If we test the magneto 
and find no spark, the next thing we should examine after test- * 
ing our ground wire should be the breaker points. See that 
they open the correct distance for that particular kind of a 
magneto. (Note. —The Bosch magneto requires .015" gap be¬ 
tween the breaker points; the Berling requires .016" to .20", 
and the Dixie requires .020".) The breaker points must open 
the correct gap, no more and no less. Any change in the ad¬ 
justment of the breaker will cause the break to occur with the 


AIRPLANE MOTORS. 


61 


armature a different distance from the pole pieces, and may 
have considerable effect on the working of the magneto. The 
breaker points should be clean and smooth. If we find them 
badly burned we can true them up with a jeweler’s file and re¬ 
adjust them so that they open the correct distance or separate 
the correct distance. However, if they are badly burned, we 
should endeavor to get another magneto, because the fact that 
they are burning shows that the condenser is not working prop¬ 
erly. 

13. I have found by experience that we can tell more about 
the actual strength of a magneto by the appearance of the 
spark-plug points than by any other means we have on the field. 
The correct distance between spark-plug points for Bosch mag¬ 
neto and Dixie magneto is 1/64" or .015" ; for a Berling mag¬ 
neto, .030". As long as these spark plugs are properly ad¬ 
justed, if the magneto is up to full strength, the heat of the 
spark will burn these points to a whitish appearance between 
the tips, and when the magneto begins to get weak from any 
cause it will fail to burn the plugs in this manner. Also, if 
the spark-plug points are too wide apart, this whitish appear¬ 
ance will not be noted. I have found that by carefully watch¬ 
ing the spark-plug points I have often been able to remove a 
magneto which was becoming weak before the engine had 
missed a shot, in this way preventing trouble. Also, suppose 
that in an eight-cylinder motor seven of our spark plugs show 
this white appearance between the tips and the eighth does not. 
If all the plug points are adjusted equally, this will indicate 
that the insulation in this spark plug is defective, and usually 
if you will break open such a spark plug you will find a crack 
in * the porcelain which will usually be blackened by smoke. 
This will prove that you were right in removing the plug. 

14. Remember that in ignition work it is the heat of the 
spark rather than the size of the spark which counts. If you 
are out somewhere with a motor and find that the magneto is 
becoming weak and have no replacement for it, you can gener¬ 
ally cause it to burn the spark-plug points white by closing the 
points together slightly; for instance, closing them in to .010" ; 
in this way you will be able to keep the magneto in commission 
for several hours longer, sometimes six or ten hours longer. 


62 


AIRPLANE MOTORS. 


Another method of keeping the magneto in commission when 
it begins to weaken is to retard the breaker housing slightly. 
This applies to the Bosch and Berling magnetos and will often 
enable you to get home with a motor or machine. You must 
not retard the spark too far or overheating of the motor will 
result. 

15. Remember that magnetos give trouble mostly at high speed , 
not at low speed, and when you can get a spark from a magneto 
by testing it in the manner I have described the motor will' 
usually start well and run all right up to about one-third of its 
speed, but may not run at all at high speed. In other words, 
a magneto will start a motor unless it is completely “ knocked 
out.” When ordering a new magneto it is necessary to state 
make and type of magneto and number of cylinders on the 
motor. Also it is necessary to state whether you require a 
“ clockwise ” or “ anticlockwise ” magneto. A magneto is said 
to revolve “ clockwise ” or “ anticlockwise ” as seen when looking 
at the gear or coupling end of the armature or rotor shaft; this 
does’not refer to the direction of rotation of the distributor. 

Timing the Magneto. 

16. Timing the magneto consists of three operations: First, 
we must plaee the motor or crank shaft in the proper position 
for the spark to occur; second, we must place the magneto in 
the position where it is delivering a spark; and, third, we must 
mesh the gears or connect the coupling. 

Placing the Motor. 

17. First of all, it is usual to time a magneto for the No. 1 
cylinder of a motor. This is not necessary, but it is the con¬ 
ventional practice. Therefore we will place the piston of No. 1 
cylinder on the right stroke. Naturally the spark must occur 
on the compression stroke, the only stroke when there is gas in 
the cylinder and compressed ready to fire. Now the question is, 
How shall we know when No. 1 piston is on the compression 
stroke? If we will turn the engine over in the proper direction 
or in the direction in which it runs until we see the inlet valve 
open, we know that we are at the beginning of the suction stroke, 
and when we see the inlet valve close we will know that the 


AIRPLANE MOTORS. 


63 


piston is at the end of the suction stroke and at the beginning of 
the compression stroke. Now, if we will put a rod or screw 
driver down through the spark-plug hole on the head of the 
piston, we can follow the piston as it comes up on the compres¬ 
sion stroke and find the top center or highest point of the piston 
travel. It is best to find this point accurately to measure from. 

18. Airplane motors are usually timed in full advanced posi¬ 
tion, because they are seldom retarded. In other words, the 
advanced position is the most important. Now, the question is, 
how much advance shall we give this motor, or, how far before 
top center shall the spark occur? The amount of advance de¬ 
pends on the speed at which the motor runs, also on the type of 
combustion chamber, whether the motor is of high or low com¬ 
pression, and whether there is one or two spark plugs per cylin¬ 
der. The faster the motor runs the greater advance we must 
give the spark or the earlier the spark must occur. If we have 
a compact combustion chamber, of the valve in the head type, or 
if we have two sparks per cylinder simultaneously, less time 
will be required for the flame to travel through the entire 
charge, and therefore less advance will be required. An experi¬ 
enced motor man can judge fairly closely where the spark 
should occur on any motor if he knows these conditions. But 
most men will do well to refer to the instruction book for their 
particular motor, or to get the information from a superior. 
Whenever we disassemble a motor or remove a magneto we 
must make it a point to determine by measurement where the 
spark occurs, so that we will have this information for that 
particular motor. 

19. As airplane motors seldom have fly wheels the spark 
timing is generally given in inches of piston travel. Instead of 
saying the spark should occur so many degrees before top center 
it is usual to say the spark must occur at a certain fraction of 
an inch before top center, meaning, for instance, a quarter of an 
inch before the piston reaches the top of its stroke. Then, to 
complete the process of placing the motor, suppose our instruc¬ 
tions for the motor we are working on are to set the spark 
5/16" before top center. AVe have already placed our piston at 
the top center of the compression stroke. We will now take a 
scale or rule and measure 5/16" upon our screw driver or rod 

35840°—18 - 5 


64 


AIRPLANE MOTORS. 


and make a mark. Then we will back the motor until the piston 
has gone down this 5/16". The piston in No. 1 cylinder is now 
in the correct position and on the right stroke for the spark to 
occur. 

Placing the Magneto. 

20. We must first decide which distributor contact we will 
call No. 1. Frequently a figure “1” will appear in a little 
window in the distributor cover when this contact is made. The 
breaker breaks once for each contact of the distributor. Turn 
the magneto until the distributor brush is on No. 1 contact. We 
will now turn the magneto carefully a few degrees until the 
breaker points just begin to separate. Just as soon as we can 
see that'the points are separating, that is when the spark occurs. 
Another method is to previously insert a cigarette paper be¬ 
tween the breaker points. This kind of paper is only .001" 
thick, and when it will slip out it will show us when the points 
are separated just that amount. That is perhaps the most 
accurate way to “ get the break.” We must also be sure that 
the breaker housing is in the advanced position, because we 
have no desire ever to make the spark occur earlier than 5/16" 
before top center in this particular motor. Now placing the 
distributor and breaker points we can mesh the gear or connect 
the coupling. Then our magneto will be timed. Sometimes, 
when the magneto has been accurately placed, we find that the 
gears will not quite mesh, but require turning in one direction 
or the other perhaps half a tooth. Many magnetos are provided 
with adjustable couplings, to take care of such a situation, but 
if our magneto is not so provided we will have to decide whether 
we prefer to have the spark occur half a tooth earlier or half a 
tooth later. If the motor is using a rather large amount of 
advance, perhaps half a tooth later would be better; and if our 
motor happens to carry rather little spark advance a half tooth 
earlier would do no harm. Where two magnetos are used they 
must be timed exactly alike or “ synchronized.” 

Wiring Up the Magneto. 

21. The cylinders receive the sparks in the same order that 
they get their gas through the inlet valves. When the designer 
of a motor designs the cam shaft, he arranges it to open the 


AIRPLANE MOTORS. 


65 


inlet valves and deliver gas to the cylinders in some certain 
order. For example, a 4-cylinder engine never fires in numerical 
order. It will fire either 1-2-4-3 or 1-3-4-2. If it should fire 
1-2-3-4 it would have a great amount of vibration and the 
strains on the crank shaft would be excessive. It is important 
then for the motor to have a certain firing order to make it run 
smoothly and also to make it draw gas through the inlet mani¬ 
fold without jerking the gas back and forth or causing it to 
reciprocate. If the engine fires in the same order in which the 
inlet valves open, obviously the way to find out the firing order 
of any motor is to crank it over and note in which order the in¬ 
let valves open. For example, we have a six-cylinder motor. 
We will start with No. 1 cylinder and turn the engine over until 
No. 1 inlet valve is just opening. Then we will put down the 
figure I on our paper. Now we can turn the motor over a few 
degrees and see which inlet valve opens next. It may be No. 4. 
Then the next may be No. 3, 6, 2, and 5. Remember that we 
must not take for granted that all motors of one make have the 
same firing order, because some may be right-hand motors and 
some left (normal or antinormal rotation), and also the manu¬ 
facturer may have found that he could improve the running of 
his motor by changing the firing order. The point I want to 
make clear is this: Remember the method of finding the firing 
order rather than trying to remember the firing order. It is but 
a few minutes work to turn the motor over and note the actual 
firing order, and by doing so you will save yourself trouble 
some time. The magneto delivers the sparks in numerical order, 
1, 2, 3, 4, etc., because as the distributor brush moves around it 
touches one contact after the other, but try to think of the dis¬ 
tributor terminals not as No. 1, 2, 3, but as first, second, third, 
etc. 

22. In timing the magneto,, we time it for No. 1 cylinder. So 
we can connect No. 1 terminal on the distributor to No. 1 spark 
plug, and the second terminal on the distributor will connect to 
the second cylinder in the firing order, and the third distributor 
terminal to the third in the firing order, and so on. 

Magneto-_- 1st 2d 3d 4th 

Cylinder Nos_ 14 3 2 




66 


AIRPLANE MOTORS. 


23. Above all, in timing a magneto, do not forget to place the 
engine on the right stroke. Before timing the cam shaft, the 
strokes on the pistons were merely up strokes, and down strokes, 
but since timing the cam shaft, each stroke has a name, viz, 
suction stroke, compression stroke, working stroke, and exhaust 
stroke. The spark must occur near the top of the compression 
stroke when there is gas in the cylinder and it is compressed 
ready to fire. This only occurs every alternate time the piston 
is up. If the spark occurred on the other top center no explo¬ 
sion would occur, as there would be no compression and no gas 
in the cylinder, and if you should go so far as to start the motor, 
you would not get so much as a “ shot ” out of it. 


LECTURE VI. 

SPARK PLUGS. 

1. A spark plug is composed of an outer steel shell, which 
screws in the cylinder, and a core of insulation material usually 
made of some form of porcelain with a wire running down the 
center. This wire is known as the central electrode. The high- 
pressure current or “ spark ” from our magneto is delivered to 
this central electrode and jumps from its inner end to some point 
arranged on the steel shell of the plug, which, being in contact 
with the cylinder, is grounded. When mica insulation is used 
it has a tendency to become oil soaked in time, and with the 
porcelain insulations there is danger of their cracking from 
great heat or rapid changes in temperature. The electrodes 
are usually made of some metal which does not scale easily 
under high temperatures. The scale would be in the nature 
of an insulator if it could form. Platinum, iridium, and nickel 
alloys are generally used for electrodes. The proper adjust¬ 
ment of the gap between the electrodes at the inner end of the 
spark plugs should be accurately made. Most of the high- 
tension magnetos require a gap of 1/64" or .015", and nearly 
all modern battery systems require a gap of about .030", or 
approximately 1/32". With high-tension magnetos if the spark 



AIRPLANE MOTORS. 


67 


plug points are set too close together the heat of the spark is 
greater than when the points are far apart. If the spark plug 
points are too close, the heat of the spark will be sufficient to 
fuse or melt the tip, and little globules or bubbles of metal are 
formed on the tips, which are liable to connect or short circuit 
them, and if the spark-plug points are too far apart it will be 
difficult for the magneto to throw a spark across them when the 
engine is running at low speed and the magneto also running 
at low speed. Because a spark will jump 3/16" outside the 
cylinder does not mean that it will jump that far inside. Re¬ 
member that the spark inside the cylinder occurs while the gas 
is compressed to 90 or 100 pounds to the square inch and under 
these conditions can jump only about one-third as far as it 
can jump outside the cylinder where the air is at atmospheric 
pressure. 

2. I have pointed out that it is the heat of the spark rather 
than the size of the spark which counts and also that the 
magneto makes a hotter spark when the points are close than 
when they are far apart, and there are other arguments in favor 
of having the plug points close together. If the plug points 
are far apart, the resistance to the flow of the current will be 
high and it will not require very much dirt or oil on the surface 
of the insulation to provide an easier path for the current than 
jumping the gap. In other words, with the plug points farther 
apart, the plug will be more easily short-circuited. I have often 
seen spark plugs with cracked insulation begin to miss at high 
speed and by taking those plugs and putting the points closer 
together, thus reducing the resistance at the points, the cur¬ 
rent would once more jump the gap and the cylinder continue to 
fire. Under these circumstances, the plug would fire until suffi¬ 
cient carbon accumulated in the crack in the porcelain to pro¬ 
vide an easier path for the current than the reduced gap in the 
plug. It is always best to adjust spark plug points accurately 
to a gage, because in a multi-cylinder motor the heat of the 
spark should be uniform in all cylinders. 

3. Occasionally we find a motor, or one cylinder of a motor, 
which floods with oil and continually fouls or short-circuits 
spark plugs. Under these conditions, a single pointed spark 
plug should be used, or if you are using a three-point plug in 


68 


AIRPLANE MOTORS. 


all the cylinders, break off two of tlie points in this particular 
cylinder. The reason for this is that in a three-point plug, while 
the spark is jumping between a certain pair of points and burn¬ 
ing them clean, oil and carbon are accumulating between the 
points where no spark is occurring. In a single-point plug 
there is a strong tendency for the spark to keep the points clean, 
due to its great heat. Also the points should be close together 
and in this way the current will be able to jump through the 
oil instead of flowing through it. In spark plugs it is a great 
problem to carry the heat away from the central electrode fast 
enough so that this part will not become red hot or incandescent. 
In some spark plugs the points do become red hot and cause pre¬ 
ignition. This will usually show up as follows: You can start 
your motor on a test block or in a machine and it will run 
smoothly for two or three or possibly ten or fifteen minutes, 
and then begin to jerk or jar a little bit. In many cases, it 
will begin to shake steadily and regularly appearing as execes- 
sive vibration. Usually the speed of the motor will be slightly 
reduced. The thing to do in a case like this is to experiment 
with some kind of spark plug having shorter and thicker elec¬ 
trodes. 

Care of Spark Plugs. 

4. When we remove spark plugs for cleaning we should gently 
scrape the carbon away, being careful not to wedge anything 
between the shell of the plug and the insulation, because we 
might crack the insulation in this manner. Adjust the spark 
plugs by your gage and note the appearance of the points. 
They should be burned whitish in color between the points. If 
they are not, it probably indicates that the points are set too 
far apart or that the magneto is weak. If one plug in the set 
fails to show this whitish appearance it should be replaced, be¬ 
cause the insulation is probably defective. If you find small, 
metallic beads or bubbles on the points they should be adjusted 
.002" or .003" farther apart. 

5. It is well to use new spark-plug gaskets frequently. By 
so doing it will be unnecessary to screw the plug in very tightly 
and the danger of straining or cracking the insulation by pulling 
hard on the shell of the plug with your wrench will be greatly 


AIRPLANE MOTORS. 


69 


reduced. Never drop spark plugs, because there is danger of 
cracking the porcelain; and always shake them to see if you 
hear any rattle in the porcelain. Unfortunately there is often 
no way in which we can detect the crack in the plug unless it 
is by noting that the spark fails to burn the points white or 
that the cylinder misses at high speed. It is an excellent plan 
to put a few drops of oil on the spark-plug thread before screw¬ 
ing it into the cylinder. If you ever have one stick in a 
cylinder you will never fail to do this. 


LECTURE VII. 

INSPECTION OR “ PREVENTION ” OF TROUBLE. 

1. In airplane-motor work if the motor fails the aviator 
usually fails to accomplish whatever he started out to do. This 
means that it is important to prevent trouble by means of 
careful planning, thorough inspection, and forethought, rather 
than to wait until the trouble occurs and then figure out a way 
to fix it. Inspect in a systematic way. Begin by inspecting 
the gas feed. To do this, first examine the gas tank and see 
that there is a good air vent and see that the air vent is not 
plugged up, because gasoline will not flow from the tank very 
long unless air can get in to take the place of the gasoline. 
Next, test the flow from the pipe where it connects with the 
carburetor. Clean the settlers and strainers in the gasoline 
line. Clean the carburetor thoroughly, including the jets. Then 
examine the intake manifolds for air leaks or loose joints. It 
is important to examine the throttle controls. The best way 
is to have someone sit in the pilot’s seat and operate the 
throttle, then stand where you can see the carburetor and be 
sure that when he operates the throttle that it opens all the 
way and be very sure that when he closes the throttle it closes 
all the way. This is exceedingly important because it is quite 
possible to install a new motor or carburetor in a machine and 
the throttle controls may not fit. The result may be that the 
throttle will not come anywhere near closing. Then when the 



70 


AIRPLANE MOTORS. 


mechanic cranks the engine the machine may start ahead so 
that the propeller will Strike him. Next let us inspect the 
ignition. See that the ground wire or short-circuit wire con¬ 
nections are tight and examine its condition throughout its 
entire length. See that it does not "vibrate or chafe against 
any metal parts, because after the insulation wears through, 
the wire will short-circuit against metal parts of the machine, 
and the effect will be the same as having the switch closed all 
the time. Sometimes a nail or tack will have been driven 
through the ground wire accidentally. This will do the same 
thing. See that the connections of the wire are firm and not 
likely to work loose, because if they should work loose the 
pilot will have no way to stop the spark occurring in the cylin¬ 
ders, making it necessary to shut off the gasoline to stop the 
motor and making it dangerous for the man who cranks the 
propeller. 

2. Examine the switch; see that there are no loose parts. 
Sometimes trouble will occur from a strand coming loose on a 
wire where it is attached inside the switch, and these strands 
may touch the opposite part of the switch and short circuit it. 
Sometimes the switch will become corroded so that it will not 
make a contact and the motor will fail to stop when we close 
the switch, or it might be dangerous to the man cranking the 
propeller. Always remember that on Bosch and Berling mag¬ 
netos, and most other makes, when the breaker cap or cover is 
removed so that you can see the breaker points operate, it will 
be dangerous to crank the motor, because your contact for short 
circuiting the magneto is usually in this cap or cover. There¬ 
fore, if you wish to crank the motor to see how the breaker 
points operate, always remove the connection between the dis¬ 
tributor and the collector brush or remove the spark plugs. 
Next we can clean the magneto. First clean the breaker points. 
The breaker should not be adjusted unless they are considerably 
wrong, and, if possible, they should be trued up with a jeweler’s 
file when readjusted. The reason is because after the points 
are adjusted they seldom come together true, and must be 
squared up with a file. The best policy is to leave the breaker 
points alone in most cases, but if you find them burned badly 
or find that they are badly in need of adjustment, it must of 


AIRPLANE MOTORS. 


71 


course be done. Clean the distributor; the best way to do this 
is to wipe it with an oily rag and then wipe off all the superflu¬ 
ous oil with a dry rag, leaving it slightly oily looking. Exam¬ 
ine the entire magneto from the outside and see that everything 
is tight. Next remove the spark plugs. Clean and adjust the 
points and see if they are receiving the proper strength of 
spark. See if they are burning white. Be sure to examine the 
plug for defect or crack in the insulation. Examine the sec¬ 
ondary wire, running from the distributor to the spark plugs, 
and see that they are not chafing against any moving parts, 
such as the rocker arms or push rods. 

2. Remember that these wires carry high-pressure current 
and any injury to the insulation is likely to cause a leak or 
short through the frame of the motor. It is well to avoid run¬ 
ning these wires close to metal parts, because the rubber in 
time becomes cracked by the weather, and if we have a foggy 
morning, making things damp, the current may leak through 
these cracks and jump to the cylinders or ground. Next we 
can inspect the valve-operating gear. In high-speed motors 
all parts of the valve-operating gear' must work freely. There 
must be no rubbing or binding. Remember that the valve 
springs must overcome the inertia of the heavy valves and re¬ 
turn them to their seats in an extremely short space of time. 
When the motor is running fast they must also overcome the 
inertia from the push rods and rocker arms or other parts of 
the valve gear and return them to their normal positions. It 
requires a surprising amount of power to do this, and if there 
is any excess friction or binding in the valve-operating gear 
the spring will fail to do it, and inaccurate valve action will 
result. 

3. Cam followers are usually shaped on the end in such a 
way that they must be held in a certain position to ride the 
cams properly. Frequently they are held by some form of set 
screw, and therefore it should be inspected from time to time 
to see if it is wearing. If a cam follower should turn around 
it will usually ruin both the cam followers and, what is worse, 
the cam shaft. 

4. The tension of the exhaust valve springs must be correct, 
or they are liable to become weakened due to their great ten- 


72 


AIRPLANE MOTORS. 


sion in some cases, and also due to the fact that they are sub¬ 
ject to considerable heat in some motors. In one motor that 
I know of, if an exhaust pipe gasket blows out, the exhaust 
flame will come in contact with the exhaust spring, thus draw¬ 
ing the temper from the steel and allowing the spring to col¬ 
lapse. These springs are always tested when the motor is 
being overhauled, but sometimes they weaken while the mbtor 
is in the machine, and can usually be detected by running the 
motor at its slowest idling speed and listening. The valve 
with weak springs will usually suck open, or open automatically 
during the suction stroke and will usually make a buzzing 
sound as it does this. Sometimes it can be detected by putting 
the finger on each exhaust valve spring in turn and the one 
which is sucking oft its seat will be vibrating in a peculiar 
manner. When these exhaust valve springs become weak, air 
will be drawn through the valves during the suction stroke 
and will flow into the cylinder and also into the intake mani¬ 
fold and spoil the mixture for all cylinders on that part of the 
manifold. Its action is very similar to that of a loose joint 
admitting air in the manifold. On the Curtiss eight-cylinder 
one hundred horsepower motors a pull-down spring is used 
for opening the inlet valve. This spring must be at least ten 
pounds heavier than the intake valve spring or else the valve 
will not open properly and a “ clicking ” sound will be heard 
while the engine is running slowly. 

5. Then, next, we should adjust the valve clearance. The 
designer of engines decides that he wishes his valve to operate 
with a certain clearance. He designs the cams to give the 
correct valve timing with this clearance. Then the engineers 
who test the motor at the factory find what clearance they 
must give the valves when they are cold, in order to have the 
correct clearance when they are hot. This clearance is pub¬ 
lished for all motors, and we should not adjust the clearance 
on any motor unless we know what the correct clearance is. 
Having found out what the correct clearance is for our motor, 
the next important step is to place the motor correctly for ad¬ 
justing clearance. This must be done for each cylinder in the 
motor. The best way to do this is to crank the motor in direc¬ 
tion of rotation until the inlet valve of the No. 1 cylinder is 


AIRPLANE MOTORS. 


73 


just closed, then turn the propeller 90 degrees further. This 
will place the cam followers on a neutral part of the cam and 
we can now adjust the clearance. This applies to all four-cycle 
engines, no matter what the type. Always use a thickness 
gauge or “ feeler ” for adjusting these clearances. When you 
realize that a thousandth of an inch variation in valve clear¬ 
ance may make four or five degrees difference in the timing, you 
will see the importance of adjusting valve clearance accurately 
and uniformly. 

6. Next, we should try the compression of the motor to see 
if all the valves are seated properly. The best way to do this 
is to remove a spark plug from each cylinder. Then put one 
in one cylinder at a time so that there will be no doubt as to 
which cylinder is on compression. One good way is to swing 
the propeller up against the compression hard enough so that 
the compression will bounce it back again several times, to see 
how many times it can be done before the air has leaked by 
the piston or valves enough to let it turn on over freely. 

7. Leaky exhaust valves can frequently be heard by listen¬ 
ing at the exhaust pipes. Leaky inlet valves can sometimes 
be heard by listening in the carburetor, but when trying this, 
have the carburetor or throttle partly open, because many 
carburetors make a wheezing sound when the throttle is closed. 

8. Then examine each part, nut and screw, for tightness. 
For instance, examine every cylinder nut; then each lock-nut 
on valve adjustments, and so on, in a systematic way, otherwise 
the loose ones will be overlooked nine times out of ten. Exam¬ 
ine the engine bed bolts to see if they are tight and locked. 
Examine the water connections. See that there is no chance 
of the hose sucking shut in the line between the bottom of the 
radiator and water pump. Examine all copper tubing and 
brass tubing carrying oil, air, or gasoline; see that they are free 
from vibration or swinging. See that they are not pinched or 
likely to be; see that they can not be chafed by any part. For 
instance, where the pipes run through an aluminum bulkhead 
in the machine, sometimes a hole is cut small and then the pipe 
will rub against the edge of the aluminum. In this case, there 
is danger of the aluminum wearing through the copper pipe 
in time. 


74 


AIRPLANE MOTORS. 


9. Inspect the Tachometer drive. Inspect the propeller. See 
that it is on tightly and locked securely. The entire power out¬ 
put of the engine is transmitted through the propeller hub, and 
unless it is very tightly set on the shaft it will “ work ” or 
vibrate and it is liable to shear the keys or loosen the hub. 
This should be very carefully watched. A rapidly revolving 
propeller has as much ability to do damage as a large charge of 
dynamite and should certainly be watched. 

10 . Never “ rock ” the propeller as a preparation to the pull 
for starting. Many men are hurt in this way. Place your pro¬ 
peller where you intend to start the pull, raise up on “ tip-toes ” 
and start your pull strongly . Remember you must pick up a 
great deal of speed in the first few’ degrees in order to “ carry¬ 
over ” the spark. As you finish the pull or stroke, manage so 
that you will be withdrawing your hands. If the motor “ kicks 
back ” never try to resist it, simply withdraw your hands in¬ 
stantly. Never have tools in your pockets while cranking, they 
may fly out of your pockets and be “ batted ” through your legs 
by the propeller. 

11 . The man at the propeller and the man at the switch 
should “ sound off ” what they are doing, so there will be no 
misunderstanding. Make it a rule that the man at the switch 
will not say “ closed ” until the switch is closed, because if he 
should say closed, and then leisurely reach over to close the 
switch, or make the motor safe, the man at the propeller might 
work faster and pull the propeller, believing the motor to be 
safe, and get a severe kick. 

Propeller Notes. 

A system for diagnosing trouble or “ trouble shooting.” 

1. When a motor is not working right w’e must first classify 
our trouble or decide what the motor is doing. For example, 
does it miss, cut out, slow down, back-fire, fail to start, fail to 
stop, or what? Then, at what speed does the trouble occur? 
Many troubles show up only at certain speeds, and therefore 
we should notice at what speed the trouble occurs. 

2. Next, if possible, notice what part of the motor is giving 
trouble. For example, on a “ V ” type motor, is the right side 


AIRPLANE MOTORS. 


75 


missing or the left side? Notice which cylinder is giving the 
trouble. Knowing which cylinder is giving trouble, we can find 
the trouble comparatively quickly. Some kinds of trouble can 
cause the entire motor to give trouble, but could not cause just 
one cylinder to give trouble. These could be called “ general ” 
troubles. Another kind can cause one or two cylinders to miss 
or give trouble but could not cause a general trouble. These 
we call “ local ” troubles. 

3. For example, suppose we say our motor is missing. Now, 
in addition to noticing at what speed the motor is missing, let 
us note the nature of the miss. For instance, is one cylinder 
missing occasionally, one cylinder missing steadily, one side of 
the motor cutting out, or the whole engine cutting out, as it 
would do if we intermittently pulled the switch? Or is it a 
scattering miss, which means first one cylinder on one side and 
then another cylinder on the other side missing, but not regu¬ 
larly? Suppose we decide that we have one cylinder missing 
occasionally. Some of the likely causes of this trouble would 
be trouble from oil, slightly cracked porcelain in spark plug, 
parts binding in the valve action or need of adjustment in the 
valve clearance; it might be water on the spark plug. Suppose 
we decide that we have one cylinder missing steadily. This is 
likely to be caused by one of the secondary wires or leads, run¬ 
ning from the distributor to the spark plug, being disconnected 
at either end; a broken spark plug, or a burned exhaust-valve 
spring, etc. Suppose we decide that one side of the motor is 
giving trouble. If this trouble occurs at high speed, it is most 
likely to be caused by magneto troubles (this can sometimes be 
remedied by slightly retarding the magneto) or it may be an 
inlet valve stuck open. If the trouble occurs at low speed, it 
is pretty sure to be caused by an air leak in the inlet manifold 
on this side of the engine, a weak exhaust valve spring, which 
has the same effect, or possibly an exhaust valve held or stuck 
open. 

4. Magnetos will cause one side of a “V” type motor, or 
every alternate cylinder in a six-cylinder motor, to cut out at 
high speed, and this is sometimes caused by the breaker points 
opening a different distance on the two cams; sometimes by 
the weakening of the magnets, due to vibration or other causes, 
and sometimes by wear in the armature bearings. 


76 


AIRPLANE MOTORS. 


5. Suppose we decide that our motor is “ cutting out.” Now, 
this is a “ general ” trouble and is likely to be caused by some 
troubles in the common source of gas. For instance, in the gas 
feed, by an obstruction in the spray nozzle or water in the gaso¬ 
line. Or the trouble may be in the source of ignition, not in the 
distributor where the current is handed out to the individual 
cylinders, but probably in the primary circuit in the magneto. 
It may be in the secondary circuit, but if it is, the trouble will be 
between the armature and the distributor. 

6. Occasionally an inlet valve stuck open will cause such a 
violent back-fire as to make the w T hole motor “ cut out,” because 
when a back-fire occurs it burns the gas in the manifold and 
the other cylinders draw a mixture wilich is partly burned gas. 

7. Now, we may decide that we have a “scattering miss” 
in our motor. This is harder to find, and frequently we will 
catch one cylinder in the act of missing and believe we have 
found a local trouble. This scattering miss is most likely to 
be caused by a weak magneto, if it occurs at high speed; but 
it can mean many things. It is usually a combination of 
several slight disorders. If we find that we have a scattering 
miss at high speed, perhaps the best thing to do is to examine 
the spark-plug points to see if they show the proper heat or 
strength of spark, and if they do not we should put on a new 
magneto, if possible. If a magneto is not obtainable, we can 
either close the spark-plug points together a few thousandths 
of an inch, or, in extreme cases, we may slightly retard the 
magneto, which will help in some instances. But if we find 
the appearance of the spark plug such that we may decide the 
spark is all right, then the best thing is thoroughly to inspect 
the motor as I have described. 

8. To find out which cylinder is missing (if our motor has 
open exhaust ports or exhaust pipes for each cylinder), the 
simplest w r ay is to hold a stick in front of each exhaust in turn. 
In this way, we can readily see when a cylinder misses explo¬ 
sions. Frequently we can watch the appearance of the exhaust 
as it comes out of the pipe, and at the same instant that we hear 
the motor miss we may see a slight difference in the exhaust, 
lighter or darker in one cylinder. This will probably be the 
cylinder that is missing. 


AIRPLANE MOTORS. 


77 


9. When seated in an airplane with a “ V ” type motor, it is 
a good plan to listen intently to the sounds coming from the ex¬ 
haust on one side of the engine at a time. With practice you 
can concentrate on the sounds coming from one side, and in this 
way you can tell a great deal more about the operation of the 
motor than by listening to the entire jumble of sounds. 

10. Sometimes when looking at the exhaust of a motor you 
will see more flame in one cylinder than in another, or different 
colored flames. This may be an indication of air leaks in the 
manifold or weak exhaust-valve springs, but it is quite likely 
to mean that the motor is cold or that the manifolds do not 
distribute the gas evenly. This is something that a mechanic 
on the field can not correct. It is well, however, to watch the 
exhaust of your motor each day and if a change takes place— 
if, for instance, one side becomes clearer and the other side 
darker, it may indicate that an air leak has developed or that 
there is a weak exhaust-valve spring. White smoke is always 
due to an excessive amount of lubricating oil, but always re¬ 
member that oil on a plug can be the cause of a cylinder missing 
by fouling the spark plug, or it can be an effect. In other 'vyords, 
if a spark plug broke and failed to deliver a spark, oil would 
accumulate in the cylinder and not be burned, and in this way 
would be an effect and not a cause. 

Troubles and Some of Their Probable Causes. 

11. Signs of overheating. 

(1) Slowing down. 

(2) Radiator steaming. Possibly we notice a smell of burn¬ 
ing oil or hot rubber. Frequently as the motor begins to over¬ 
heat, vibration will increase, making the motor seem to run 
harshly. Sometimes a smell of too much gas or smell of rich 
mixture will be noticeable. In any case, stop the motor as 
soon as you can when this is noted, because continuing to run 
a motor when it is overheated will usually ruin it. When feel¬ 
ing the motor to see if it is overheated, remember that it must 
be felt immediately after stopping the engine, because after 
the motor is allowed to stand a few minutes the outside be¬ 
comes considerably hotter. The reason for this is that the 


78 


AIRPLANE MOTORS. 


water does not circulate when the motor is stopped and be¬ 
comes highly heated, in turn heating the outer wall of the water 
jacket and making the motor seem very hot. The radiator, if 
felt immediately after stopping the motor, will probably feel 
about “blood warmth” at the bottom and quite hot.at the top, 
but after standing for some time, it may be quite hot all over. 

Some of the Causes of Overheating. 

12. Some of the causes of overheating are: 

I. Obstructed water connections. For example, dirt or rust 
flakes in the pipes. Or frequently in using hose connections 
when the pipe is thrust into the hose, the end of the pipe will 
catch and “ curl ” the inner layer of fabric back into the hose; 
in this way at least partially shutting off the water. 

II. Broken piston rings. This will cause overheating by allow¬ 
ing the hot gases to pass down between the piston and cylinder 
walls. 

III. Leaky radiator. 

IV. Oil in the radiator. This will cause overheating by pre¬ 
venting the water from coming in actual contact with the metal 
of the radiator, which is cooled by the air. If we take two 
pieces of iron and heat them to a black heat and put oil on one 
of them, then drop both of them in a pan of water, we may be 
able to pick the clean one up immediately, but the one which 
is oily will remain hot a great deal longer. Therefore, never 
use an oily measure to fill your radiator. 

V. A rich mixture. 

VI. Early or late sparK. The rich mixture and the late spark 
cause overheating by causing the maximum temperature to 
occur late, at a time when there is a great amount of cylinder 
wall exposed to absorb heat, and therefore this surface will be 
harder to cool. 

VII. Failure of oil pump or oil supply. This causes overheat¬ 
ing on account of excessive friction in dry bearings. 

VIII. Tight motor or new parts. If the bearings are tight 
in a motor or there are some tight-fitting parts, the extra fric¬ 
tion may cause overheating. 

IX. Hot weather. The cooling systems on airplanes are as 
small and light as possible, and while they serve to cool the 


AIRPLANE MOTORS. 


79 


motor in reasonable weather, they may give trouble from over¬ 
heating on exceptionally hot days, simply because the size of the 
radiator and the amount of water is not sufficient for these 
extreme conditions. 

X. Excessive deposits of carbon or any other cause of pre¬ 
ignition will cause overheating. 

13. Causes of loss of power: 

I. Preignition. Overheating causes loss of power by causing 
preignition in many cases. Remember that preignition means 
ignition occurring from some hot parts of the cylinder or hot 
particles of carbon, and occurs far earlier than the spark would 
occur, causing a tendency for the motor to work against itself. 

II. Tight motor. 

III. Bad valve or spark timing. 

IV. Broken piston rings. 

V. Stoppage in fuel line or carburetor. 

VI. Waste of rags in the carburetor. Motors in airplanes 
draw a large amount of air through the carburetor, and if a 
bunch of rags were carelessly left under the hood it would 
easily be sucked up by the carburetor and would be liable to 
make serious trouble. 

VII. Leaky valves or need of overhauling. 

VIII. Weak valve springs. 

IX. High altitude. When a motor is running in high altitude 
the atmospheric pressure being very much less than at sea level, 
the piston will not draw a complete charge. In other words, 
there is less difference in pressure inside the cylinder and out¬ 
side, therefore less air will rush in through the carburetor, and 
the motor never shows as much power in high altitudes as at 
sea level. This can be partly corrected by opening an auxiliary 
valve in an effort to increase the amount of air flowing into 
the cylinders. 

Vibration. 

14. Some of the causes of vibration are: 

I. Preignition. 

II. Air leaks in the inlet manifolds. This causes uneven 
mixture in the different cylinders and therefore a different 
action in the different cylinders and uneven explosions. 

35840°—18 - 6 



80 


AIRPLANE MOTORS. 


III. Weak valve springs. These cause vibration by admitting 
air in the same way as the intake manifold leaks. 

IV. Broken piston rings. These cause uneven compression. 

V. Rare mixture. 

VI. The magneto breaker housing may be put on crooked or 
worn, so that the breaker .separates different distances on the 
two cams, causing uneven spark timing. 

VII. The propeller may be out of balance, warped, or “ flut¬ 
tering.” If the propeller is built too lightly, it will flutter. This 
can not be corrected. 

VIII. Engine-bed bolts loose. 

IX. Engine parts of uneven weight. For example, some pis¬ 
tons heavier than others. These parts must be as nearly as pos¬ 
sible the same weight, usually within one-quarter of an ounce. 

Loss of Compression. 

15. Some of the causes are: 

I. Valve held open, or no clearance. 

II. Leaky valves or valves needing grinding. Sometimes the 
valves have chunks of carbon holding them off their seats. 

III. Scored cylinders. 

IV. Cracked pistons or cylinders. 

V. Worn or broken piston rings. 

VI. Cylinder dry or out of oil. For example, if our motor 
has been standing in the storeroom without being run, all the 
oil may drain off the pistons and cylinders, leaving them com¬ 
paratively dry, and poor compression will result. Sometimes 
in priming a motor with gasoline on a cold morning we will get 
an excess of gasoline in the cylinder and wash the lubrication 
out of it. In either case the remedy is to put in some extra oil 
through the spark-plug hole. 

VII. Loose spark plug or bad spark-plug gasket. 

Failure to Throttle Down, or Stopping When Throttled. 

16. Some of the causes are: 

I. Throttle may be stuck or shifted in the carburetor. 

II. Throttle controls may be adjusted wrong, not allowing the 
carburetor throttle to close. 


AIRPLANE MOTORS. 


81 


III. Very bad air leaks in manifold or weak exhaust valve 
springs. In these cases the excessive amount of air leaking in 
will either stop the motor when it is throttled down or cause it 
to idle too fast. The latter trouble would be in case the mixture 
had been too rich. Then the excess of air would thin this mix¬ 
ture down and make more of it. The result would be the motor 
would idle too fast. The best way to find air leaks in the inlet 
manifold is to run the motor throttled and put oil on each point 
with oil can. It will be sucked in where the leak is. Don’t use 
gasoline, because there is danger of fire from the exhaust. It is 
possible to get enough suction for this test by closing the throttle 
and turning the motor over by hand. 

IV. Spark plug points may be much too wide apart. In this 
case it is difficult for the magneto to throw a spark across the 
points at low speed. 

V. Throttle closed too far. Nearly all carburetors have an 
adjustment to limit the closing of the throttle, usually on the 
throttle arm. 

VI. Too much or too little gas, or, in other words, imperfect 
low-speed gas adjustment. 

VII. Auxiliary air valve may be loose or stuck open. 

Back-Firing. 

17. A back-fire is caused either by the inlet valve being held 
open, spark occurring while the valve is open, or by a rare 
mixture. 

18. Let us take these causes one at a time and reason them 

° U If the inlet valve is held open, that might be caused by lack 
of clearance, cam followers stuck, or valve stuck open. If the 
spark is occurring while the inlet valve is open it might be 
caused by the magneto being timed wrong, the valve timed 
wrong, or the magneto leads (secondary wires) being crossed 
or connected to wrong cylinder. If the back-fire is caused bj 
a rare mixture, remember that the rare mixture may be from 
either too much air or not enough gas. If it is too much air 
the auxiliary air valve may be stuck open or the spring may be 


82 


AIRPLANE MOTORS. 


weak. You might have air leaks in the inlet manifold or weak 
exhaust valve springs admitting air. If the motor is not getting 
enough gas, this might be due to a clogged air vent in the gas 
tank, water in the gasoline pipe or in the jets. It might mean 
that the motor was cold or that hot air was - needed'. 

Loud Exhaust. 

19. If the motor is making a loud exhaust, the question is, 
What can cause it? The noise that we call the exhaust is 
pressure escaping through the exhaust valve when it opens. 
An unusually loud exhaust means there is an unusual amount 
of pressure at the time the valve opens. This might be caused 
by the valve opening while there is high pressurethat would 
mean opening early. The valve might be stuck open; a cam 
follower turned around; or, if all the cylinders are making a 
loud exhaust, it might be the cam shaft is out of time. 

20. Now, the other way to think of it is that there may be 
high pressure at the time the valve opens normally. This might 
be caused by a rich, slow-burning mixture, or from a float valve 
being held open by dirt. Possibly the rich mixture could be 
caused by the tube through which the carburetor sucks the warm 
air being sucked shut. A late spark can cause a loud exhaust 
by causing the maximum pressure to occur late in the same way 
that the rich mixture would cause it. 

21. If all cylinders make a loud exhaust, it may mean that 
the magneto is timed wrong. Sometimes a loud exhaust in one 
or two cylinders may be caused by the magneto leads or spark¬ 
plug wires being crossed or mixed up. 

Failure to Start. 

22. If the motor fails to start, the first thing to do is to inspect 
the gas feed carefully. Prove that the gas flows through the 
passage where it joins the carburetor, and prove that it flows 
through the jets by running a wire through them. If the trouble 
is not found then, test the magneto by “ grounding ” a screw 
driver against a cylinder. Place it close to the top of the col¬ 
lector brush or any place where we can get at the secondary 



AIRPLANE MOTORS. 


83 


current before it reaches the distributor to see if we get a spark. 
If we fail to get a spark, remove the ground wire and try it 
again. . If we still get no spark, thoroughly inspect the magneto, 
and if no spark can be obtained we must have a new magneto, 
or at least a magneto expert. 

23. If the magneto tests out all right, the next most likely 
cause of failure to start would be that the motor is “flooded,” 
or, in other words, in trying to start we may have drawn so 
much gasoline into the cylinders that it can not burn. In this 
case we must shut off the gasoline and close the switch, open 
the throttle, and endeavor to get a charge of clean air into the 
cylinders, or, in other words, “ air the motor out.” 

‘ 24. If the motor has been flooded, it will probably start under 
these conditions. If we have found all these things all right, 
then the trouble must be that either the spark or valve timing is 
incorrect. In that case check the spark timing; turn the engine 
over until the inlet valve on one cylinder has just closed; remove 
the spark plug and insert a screw driver or wire and follow the 
piston up to the top of the stroke; back it down from one- 
quarter to one-lialf an inch. Now, by following the spark-plug 
wire of that cylinder to the distributor we should find the dis¬ 
tributor brush touching the contact to which this wire is con¬ 
nected. 

25. If this is correct, the spark timing is not at fault, be¬ 
cause if the magneto was timed close enough so that we would 
find the distributor in contact by this method, it would be 
near enough for the motor to start, or at least for it to “ kick ” 
one way or he other, certainly on the right stroke. If we 
have time, we can measure the magneto timing more accurately 
to see if the amount of advance is correct, but it is seldom that 
we find any great mistake in the amount of the magneto ad¬ 
vance, while we often find the magneto firing on the wrong 
stroke. 

26. If the valve timing is suspected, it is only necessary to 
turn the engine over and find out by watching the exhaust 
valve, and by putting a rod through the spark-plug hole, where 
the exhaust valve closes. The exhaust valve must close on or 
slightly after the top center. 


84 


AIRPLANE MOTORS. 


Failure to Stop. 

27. Sometimes when we close the switch the motor keeps 
on running or “ kicking.” If it merely continues to kick, the 
chances are that the motor is hot, or overheated. Possibly 
some particles of carbon, spark-plug points, or exhaust valve 
remain red-hot (or incandescent) and are igniting the charges. 
If the motor continues to run smoothly after we close the 
switch it is likely that the ground wire fias become discon¬ 
nected or the switch fails to make contact. Possibly the 
breaker cap or cover is off and not making contact. Some¬ 
times the breaker housing is not put on far enough. In this 
way the breaker cover is held away from the breaker, so that 
it can not short circuit it. 


LECTURE IX. 

INSTALLATION, CARE, REMOVAL, AND STORAGE OF 

MOTOR. 

1. In placing a rope around the motor preparatory to raising 
it for installation in the fuselage, be careful to keep the rope 
away from any part it might injure. The water jackets on air¬ 
plane, engines are usually delicate, and the rope must not press 
against them. Priming cocks are easily broken off. Rocker 
arms are likely to be bent, and small copper or brass tubing 
could be smashed flat by a rope. The timbers or beds on which 
the motor will be placed in the airplane must be level and 
parallel, and the bolts holding them in the machine must be 
tight. 

2. After the motor is placed in the machine, the switch and 
ground wire should be connected up first, so that the motor 
may be made “ safe ” while the propeller is being installed. In 
making up the water connections, be careful that the ends of 
pipes do not “ turn in ” the inner layer of fabric in the hose, 
thus obstructing the flow of water. If tape is used on these 
water joints, it must be shellacked afterwards, because if the 



AIRPLANE MOTORS. 


85 


tape is not coated with shellac it will unwrap and deteriorate 
from the effects of the hot weather. Make it a rule never to 
connect the gasoline to the carburetor without first allowing 
gasoline to flow through the pipe for the purpose of flushing or 
cleaning the pipe and also to prove the flow of gasoline. 

3. Check the adjustment of throttle and spark retard wflres 
or rods to see that they are properly adjusted to the motor. 
The throttle must open fully and close fully, and when the 
magneto is pulled to the retard position, be sure to see that the 
spring will pull back into the full advance position, because 
if it fails to do this, the motor will probably overheat. 

Y 

Testing the Motor on a Testing Block. 

4. Airplane motors are tested after overhauling before being 
installed in an airplane. 

5. Around flying fields we seldom have opportunity to test 
a motor for horsepower output, but generally put a standard 
propeller on the motor or a “club” (dummy) propeller which 
will turn at the same speed as the propeller. If the motor 
turns this standard propeller or “ club ” to the required number 
of revolutions its power output is considered satisfactory. 

6. While running on the block with this propeller or “ club ” 
the motor must: 

A. —Run up to specified revolutions. 

B. —Throttle down and pick up smoothly when the throttle 
is opened. 

C. —Not accumulate oil in the cylinders or foul spark plugs. 

D. —Reveal a proper oil-pressure for that particular motor, 
so that it will not throw oil. The motor must cool properly 
and not have excessive vibration. Must not slow down after 
running half an hour or so, or seem to meet very accurate 
adjustment, and must not be “ sensitive.” Compression must 
be even and motor must turn freely. 

Care of the Motor. 

7. To give you some idea of how an airplane motor is cared 
for, I will tell what is done to one make of motor used to con¬ 
siderable extent in the Government schools. 


86 


AIRPLANE MOTORS. 


8. The motor receives a thorough and careful inspection and 
cleaning daily. After each five hours’ running the valve clear¬ 
ance is carefully adjusted to a gauge, spark plugs are removed, 
inspected, and adjusted. All the oil is removed from the oil 
pump and renewed after fifteen hours’ running. This is because 
the oil comes in contact with the hot piston heads and deterio¬ 
rates from the heat; also becomes filled with chunks of chrbon 
and a certain amount of liquid fuel condenses in the cylinder 
and mixes with the cylinder oil, impairing its lubricating quali¬ 
ties. Remember that if a motor is run too long without over¬ 
hauling, when it is finally removed from a machine it will 
require an expensive rebuilding instead of overhauling. 

Removal of Motor. 

9. Make it a rule to remove the propeller first and ground 
wire connections last. This is so that the motor may be made 
“ safe ” as long as the propeller is on it. Be sure that every¬ 
thing is disconnected before raising the motor with the “ chain ” 
fall or tackle. 

Storage. 

10. While the motor is in storage be sure there is no gas left 
in the carburetor. If gas is left in the float chamber of the 
carburetor it will evaporate and leave a deposit of some kind 
that is likely to clog the gas passages when the engine is used 
again. Oil should be put in the cylinders through the spark¬ 
plug holes and the motor turned over by hand a few times, at 
least each week, to prevent rust in the cylinders, and the valve 
stems should be oiled with an oil can. 

Overhauling. 

11. Overhauling is a very elastic word. It can mean merely 
disassembly and reassembly of a motor, or it can mean very 
carefully taking a motor apart and noticing each place where it 
is wearing or rubbing and investigating the cause, and careful 
planning to correct and improve conditions while assembling 
the motor. 


AIRPLANE MOTORS. 


87 


12. One important thing is to trace the lubrication system 
from the oil pump or through the motor and back to the sump. 
After learning the route through which it travels all passages 
should be carefully cleaned out by forcing gasoline through 
them to avoid any possibility of stoppage in the oil circulation. 

13. After a motor is overhauled it is best to make two or 
three short flights, for instance, 10 of 15 minute flights, before 
a long flight is made, and the motor should be examined or 
felt after each of these short flights. It is far better, if possible, 
to install the motor on a block and test it thoroughly before 
installing it in an airplane. 


LECTURE X. 

DIFFERENCES BETWEEN AIRPLANE AND AUTOMO¬ 
BILE ENGINES. 

1. First of all, airplane engines are always built lightly and 
this means that the crank case is not very rigid. The timbers 
or beds on which the engine sits must be carefully lined up 
and be perfectly true so that the crank case will not be sprung 
out of shape when it is bolted to them. 

2. Airplane engines run at nearly full capacity all the time. 
This means that we have full compression, maximum tempera¬ 
tures, and maximum pressures, also maximum vibration all the 
while. Lubrication of moving parts becomes a greater problem 
under these conditions. For example, in a racing automobile 
the driver throttles down his motor to a certain extent on the 
turns. This will allow the crank shaft to ease up on its bearings 
slightly, allowing the oil film to be renewed under the shaft. In 
the airplane motor there is no such easing up on turns. Air¬ 
plane engines run at high temperatures because of the neces¬ 
sarily light weight of the cooling systems. In other words, the 
cooling systems are very close to the limit. Airplane motors 
are subject to strains caused by the propeller. Even the best 
built airplane propellers vibrate or flutter to a certain extent 
and all this vibration goes through the motor. Usually the 



88 


AIRPLANE MOTORS. 


crank shaft gets it first, and when flying the gusty winds impose 
great strains on the propeller and crank shaft due to the uneven 
air through which the propeller is working. As the airplane is 
“ pushed ” or “ lead ” through the air by its crank shaft in 
most cases, it is necessary to have a substantial thrust bearing 
on the crank shaft to take care of these strains. 

3. It is somewhat difficult to tune up an airplane motor on 
the ground for the following reasons: The motor can not be 
run long on the ground without overheating because the air¬ 
plane is stationary and the propeller throws little or no air 
through the radiator. (The central part of the propeller is 
not designed to do any work or throw any air to amount to 
anything, and the result is that it throws practically no air 
through the radiator when the machine is standing still.) 
The result is, if we are tuning up the motor, we can run it with 
throttle open for short intervals. 

4. In some machines there is a difference in the atmospheric 
conditions around the carburetor when the machine is stand¬ 
ing or flying. For instance, when the machine is flying, there 
may be either a decrease or an increse in the atmospheric 
pressure, due to the location of the carburetor, and this makes 
carburetor adjustment on the ground difficult in some instances. 

5. Airplane engines must run at high altitudes and under 
these conditions encounter very low temperatures and very 
low atmospheric pressure, which means that the cylinders 
will not get a complete charge of gas. A motor loses a large 
percentage of its power, even at a 10,000-foot altitude. The 
reason is that there is less air pressure outside the motor; 
therefore less air or mixture rushing through the carburetor 
and into* the cylinder during the suction stroke. One of the 
remedies foy this is to open an extra air passage in the inlet 
manifold to allow a larger volume of air to enter the cylinders. 
Other methods are being experimented with. 

6. Airplane engines must run at all angles. This means 
there must be plenty of “ fall ” from the gasoline tank to the 
carburetor. The oil sumps of the crank case must have the 
oil continually pumped out of them because a large accumula¬ 
tion of oil in the crank case would flood the cylinders under 
certain flying conditions. Some of the exhibition flyers have 


AIRPLANE MOTORS. 


89 


had carburetors especially arranged to feed gasoline while the 
airplane is upside down. Such devices are not in common 
use, however. 

7. You probably know that if we take “ hay-wire ” or soft 
iron wire and bend it several times it becomes brittle and 
breaks. This is known as crystallization. In airplanes, if 
the engine vibrates, it sends little strains through the entire 
airplane which bend the metal parts ever so slightly. As 
these vibrations occur frequently and for long periods, we find 
the metal in time becomes crystallized in the same way as the 
soft iron wire I spoke of. The result may be that while the 
machine is “ taxying ” along on the ground to the hangar, 
fittings may snap simply because they are crystallized. I 
'mention this so that you will know the importance of keeping 
a motor running smoothly. On an airplane motor the com¬ 
pression in all cylinders must be uniform. Moving parts must 
all weigh alike. Spring tension must be uniform. Spark plug 
points must be adjusted at uniform gaps. If more than one 
magneto is used, they must be carefully synchronized or timed 
together and certainly all the plugs must be of the same make 
and type. 

8. Airplane engines must be free from vibration, because 
engine vibration rapidly crystallizes all the metal parts in the 
airplane, which means that they are liable to break without 
being subjected to any great strains. 

9. Most American airplane engines are built with the car¬ 
buretor very low. While this gives a long inlet manifold which 
has some disadvantages, it has the advantage of placing the car¬ 
buretor where it is easily fed from a tank located in the fuselage 
of the airplane. Many European machines have a gas tank 
placed in or under the top plane, but this necessitates pipes rum 
ning up to this tank and consequently increased danger from 
leaks and possible fire. 

10. A good many air-pressure, gas-feed systems are used with 
good success, and also gasoline pumps pumping an excessive 
amount of gasoline to a tank which will feed the carburetor by 
gravity. This tank has an overflow pipe to the main storage 
tank to take care of the excess delivered by the pump. Re¬ 
member that gasoline tanks in airplanes must have “baffle 


90 


AIRPLANE MOTORS. 


plates ” to keep the gasoline from washing from one end to the 
other. One reason for this is that if gasoline could rush from 
one end to the other it might seriously affect the balance of the 
airplane. Air vents in gasoline tanks must be so arranged that 
should gasoline splash out of them it could not be ignited by the 
hot gases from the exhaust pipes. Frequently these vents have! 
a tube running down through the bottom of the fuselage for] 
safety. 

11. Up to the present time most airplane motors have beeru 
equipped with exhaust “stacks” (short pipes). The purpose^ 
of these stacks is to carry the gases away from the fuselage,] 
and also to have a slight syphon effect to scavenge the cylin¬ 
ders, but at present many airplanes are equipped with pipes 
to carry the exhaust gases and sounds above the upper plane. 

12. It is difficult to use mufflers on airplane engines, partly 
due to the fact that a muffler reduces the power of a motor, but 
more particularly because an engine is more difficult to cool if it; 
is equipped with a muffler and the exhaust is more liable to burn.; 

13. The gaskets between the exhaust pipes and cylinders are 
important. If they blow out, there is danger of the fiame coming 
through and burning some part of the airplane, or in some motors 
this flame could ruin the exhaust-valve springs by overheating 
them and causing them to collapse. 

14. Gasoline strainers and settlers must be arranged so thati 
they will work properly with the airplane in various angles. 
Many settlers now in use fail in this respect. 

Back-firing. 

15. Back-firing in the carburetor is particularly dangerous in' 
airplanes. Some manufacturers are experimenting with devices 
to prevent back-firing. If an airplane catches fire, it is a very 1 
dangerous situation for the pilot, because he can not step out on 
the ground and use the fire extinguisher. If he is up in the air, 
he is likely to be burned badly before he can land the machine, 
therefore every precaution should be taken to avoid chance of! 
fire. 


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